At the risk of sounding silly, I suggest a Tower 75 (really). Aside from being cheap, it is a killer engine & the TF in-cowl muffler fits. It makes the TF Spit go like hell & it has a nice whistling howl, like a Merlin V-12 at full song.

I had (note had) a TF Spit with that combo & I am in the process of building another.

WRT the TF in-cowl muffler -- while it is sized to work with 60 - 75 size engines, it can be made to work well with a 90 after a few simple mods. You have to mill (or file if you have spare time) the header inlet opening to fit the outlet port in the engine exhaust stack, plus you have to grind, or file down the "step" inside the header outlet. Works just fine with a 90.