Hmm, I wonder if I could do something similar with my TX81Z, which is more like a DX11. But I don't really need to – it's great as it is, and shines as a 4-op FM synth because of its non-sine waveforms, and because of its multitimbral capability. Still cheap on ebay too.

Though a free FM softsynth like Dexed is hard to beat for cost and convenience; and on the hardware side, Korg's opsix has a great front panel with knobs and sliders for programming, while their Volca FM2 is a nice 6-voice mini-synth that can play DX7 patches.