It is worth noting that the cards in the expansion are hard replacements for other cards so it is easy to proxy the expansion cards with the cards they replace (there are only 4 of them). Just tell everyone that those 4 cards have different rules, and then explain the other changes to the game regarding petitions/starting hands. That is actually how this expansion came about, long before it was actually printed.
Playing with the Republic rules, players will find Glory to Rome a bit more forgiving (especially to new players) and more balanced in regard to the Craftsman/Architect roles. The Republic Expansion includes 15 cards:
Crane (x6), which replaces the Circus and allows you to use a card to lead or follow Architect.
Domus Aurea (x3), which replaces the Coliseum and lets you demolish incomplete buildings with no materials when performing the Legionary role.
Forum Romanum (x3), which replaces the Forum and allows for an instant win if you have one client of each type and one material of each type in the stockpile.
Tribunal (x3), which replaces the Senate and allows you to draw one Jack when performing the Thinker role.
When using the Republic Expansion, players start with five order cards in hand (instead of four order cards and a Jack). Players may also Petition with two cards of the same role instead of three.