Tunnels and Trolls is a tried and true game from the ancient realms of role playing itself, a simple yet elegant system with a remarkably flexible core mechanic. If you haven;t checked it out yet, you should!
Converting an historical roman republic to T&T doesn’t require much effort. I would suggest the following modifications to help you out with the flavor or your campaign, however:

Priests: The old, out-of-print Sorcerer’s Apprentice provided a priest class in issue 9/10, which I’d recommend if you can find it, but for those who don’t, assume that all Wizard class characters (and Rogue class, if you wish), can choose to voluntarily declare their magic as coming from their pious dedication to a chosen deity or pantheon. If they are ever at risk of violating the tenets of that divinity, then the character will have to make a SR at the level of the spell being cast against their CHA or risk spell failure (or backfire if the roll was especially bad). Only by serving penance or performing an appropriate task will the PC be able to fall back in favor with his/her god. As a perk to offset this limit, priest characters may perform any auguristic divining magic at one half Strength cost. Alternatively, the PC may pick a single spell at each character level which he may ask his god to grant him at half the normal STR cost. He may ask such a favor once for every three levels he has achieved.

Mental Strength (or Will): All Wizards and spellcasters should have a Mental Strength attribute added to their character, which is the actual spell point source used to casting spells, instead of he regular strength score. This will reflect the more subtle, mental effect of spells in the Ancient Realms. Roll this stat like all others. It may be leveled up as regular strength in all other respects, but is now the source of magical energy. It has no other effect.

Warriors: Traditional warriors in T&T gain an armor bonus, which actually reflects the nature of Roman gladiators very well; gladiatorial armor was showy, and not as useful as one would like, and making do with what you’ve got becomes a kind of art. To add some new combat options to the Warrior class (and the Rogue and Warrior-Wizard may use this, also), I’d suggest the following new maneuvers:
Parrying: A fighting character may declare a parry in a given round. He may now roll his weapon dice and adds, then add his personal adds to his armor hits, instead of his attack roll. At first this seems like no real effect, as it won't make much difference in one-on-one combat, but in fact what happens in group combat is the PC in question is now spending more time protecting himself then working with the rest of his fellows in battle; the chance of more hits coming through increases, and the number of hits that one character takes will tend to drop due to the extra protection. This can be thought of as skulking around the edge of the party, avoiding combat and parrying more hits.
All Out Attack: A character may declare an all out attack, in which defense is disregarded completely. To make an all out attack, take your STR adds (subtract 10 from your STR), and add that number as a personal adds bonus to your hit point total. Take that same number and subtract it from your armor hits protection. This will give your attack a boost, from the raw fury and muscle, while reflecting the diminished effect of armor worn by your crazed fighter. Your armor will always retain 1 Hit of protection, however (it can't go completely away!) At the end fo the combat, the all out attacker makes a L1SR LK, and if he fails, he takes incidental damage equal to the difference he missed the roll by (this si not absorbed by armor, and is regardless of whether the warrior won the actual attack round; it's damage from carelessness.)

Skills: Skills are not a part of the old fifth edition T&T (being a system from a simpler age), but many people like to try and jack-knife them in. If you don’t have access to the Mercenaries, Spies, and Private Eyes RPG (using the same core system, with a skill set you can adopt), then I suggest the following simple alternative:
Each character receives a number of skills/talents from a pool which is chosen according to how the player views his/her character. Each character has a number of skill points equal to his/her IQ rating (and these points therefore improve when the IQ score goes up). You may spend 1 IQ point to add a +1 to any talent/skill. Whenever a saving roll comes up in which you think your skill or talent would be useful, then you may ask the Gm to let you add it to the affected attribute as if you gained a temporary bonus to that attribute for purposes of the Save. (Ex: You have Juggling+3, and the GM has you make a Level 3 Dex SR to catch a number of falling eggs. You call it, and he lets you add the +3 to your Dex attribute, for purposes of determining the actual SR number.
So where are the long list of skills and talents? Make some up! There are countless RPGs out there with long lists. Or, just let the players call it, and the GM can rule out any talents or skills that seem excessive or too broad.

Need more incentive to check out T&T? I will soon be posting an archive of all my old T&T articles, solitaire adventures, and other goodies from the old days. Just how much good stuff can someone have for a game in their closet, you ask? Well, a heck of a lot, actually! I published a T&T-focused fanzine called The Sorcerer's Scrolls back in the late eighties while in High School, and it had a lot of support and fan interest at the time. Some of the material is worth the effort to reconstruct and archive on the web, and I am working slowly on doing so. Check back here periodically for more details!
Right now, I am working on translating all of the old solo adventures from TSS's pages that I can find. Unfortunately, I do not have access to all of the old issues of TSS that I should; if anyone out there can help me recover some of those older issues, catch me at camazotz@zworg.com.