I'm going to guess that your crankset has a 110 bolt circle diameter. Chainrings up to 54 teeth shouldn't be hard to find. If it was my bike, I'd pay the extra money for rings with the built-in shifting aids. You'll have to install a longer chain. You'll probably also have to get a road triple front derailleur. It should work fine with your present flat bar shifters. Santana, Burley etc. all used mountain shifters with road bike front derailleurs until the relatively new flat bar road shifters came out.