I have the same problem with the FFVS J-22. I've done the skins, but the original FM and DM for the model are way off, so I haven't released anything yet.
Ideally, I should contact the model maker and ask him if he'd be happy for us to fiddle around with it!
Meanwhile, if we're going to be what-iffing, I use the Firepower modified Ho-229 rather than the stock Go-229. It's the same model, but tweaked a bit. Free, on... Flightsim? Simviation? Something like that, anyway.