As I stated earlier, I have the exact same set-up and it works well. My engine is on it's side which is no problem as the mounts work well both ways. The Muffler will extend down and work, I do not have the engine cowl on...seems cowls invite problems requiring removal right after installation. I plan on a Pitts Muffler.
Running right the ST 90 will work well with the 15x4 APC wide blade. Before going and spending a small fortune on a YS or other engine I at least would try and work out the existing problems.