All you need is a new crank, and the proper BB to go with it. "Compact" cranks are nothing more than a 110 bolt circle diameter (BCD) crankset. It's been around a long time, so it's not special to any one or two brands..... it's universal.(Except some Campy stuff, they are always doing something to the rings to make them proprietary!) You can use any brand with you shifters, cassettes ect. Brand makers want you to believe you need to stay with the same brand for everything, but they just want to sell you their stuff.

A 110 BCD can take rings from 34 teeth all the way up to 58 teeth. Compact cranks are often sold in a 50/34. You are certainly not limited by those rings though. Replacement rings can be from any brand are can be had inexpensively. With 11 tooth cassettes these days you can even get by with a 46 or 48 depending on your needs.

Here's a real nice 48/34 Sugino crank for instance.... http://www.velo-orange.com/suxddocr48.html from this selection of cranks....http://www.velo-orange.com/cranksets.html They take square taper BB's. Easy to find, inexpensive. For those that may not know....Sugino has been making cranks since the 70's. A very very good brand.

Many choices beyond Shimano and the other big brands!!