Yamaha DX-7

To build a 6 operator fm synth similar to DX7 should be easy enough. To build a DX7 model that is accurate enough to play original patches would be possible, but much more difficult. Best starting point I think would be to work through the Dexed source code on github. There is plenty of stuff on the web about DX7 modelling too.

For a more generic 6 op fm synth, use the library FM operators (core library). The only tricky bit is building the envelopes, but that's not too bad really. Probably linear sections, then process them through some log mapping curve. Then after that, working out how to bring all the parameters through to a GUI. Most of the difficulty there is interface design rather than implementation I think.

It really depends how close you want to get to a DX7