Diamond Dust
I've got a D.D. as well, actually just building 3rd one now. What I do is replace engine mount with 1/4" ply and add 1/8" lite ply doublers from front to just behind the front spars. I'm using a Rossi quicke engine and pipe with 8-8 APC prop. Haven't had a radar gun on it but timing on known distance -lenght of flying field- puts it at 180-190 MPH. What a blast!! Good luck with yours.
Don