Obviously "porting/polishing etc" could achieve an awful lot, bearing in mind that prior to the introduction of kit cylinders into BSSO Group Four, 25 BHP was the norm. Virtually all were fan cooled
There must be loads of such top ends laying about unused, in which case, the hard work will have been done. You could do worse than post in the 'Wanted' section. I know of one ex-race scooter locally that is as reliable as any other road scooter, even with it's (proper) flat slide Dell'Orto & a nicely muffled ex-race exhaust. Possibly one of the ultimate street sleepers with a massive grin factor.
As you must surely be upgrading the crankshaft, a longer stroke would enable the softening of the tune if the above concept scares you