ORIGINAL: NM2K
To be honest with you, the YS .45 two-stroke and the .63 to .70 four-strokes would be my choices. Especially the four-strokes. The four-strokes by YS are unmatchable by any two-stroke in the .40 to .52 displacement range. I'm not referring to just horsepower, but tractability also.
Ed Cregger
Right On!! The current YS-70 four stroke would be a great power plant!! Using the Hatori wrap around header and short pipe makes it a nice rear exhaust set-up...Great suggestion Ed!!
Chuck