Moe,
I do have the gas MVVS too. It is very good. I get close to 8000 tmp on a 18*8 Menz prop.
But this engine needs a pipe, have a krumscheid pipe on it. A 16 inch prop is to small for this engine.

But as mark said the answer to your problems is not to fit in a heavier engine.
Make the MVVS work better or use a OS FX1.6