Re: +7 mph by fixxing poor quality on 66cc
My 2cents would be with kcv regarding splitting the cases of a new engine just for basic inspection.
From the many, many threads and posts I have reviewed here on the forum, and my limited experience with this flavor of engine, there have been a very limited number of problems with the crank and rod assembly.
SO, the odds are in your favor, in that respect. lol
There are a number of probs reported regarding the crank seals.
However, I would NOT advise pulling the pinion, or the rotor, on the opposite side of the crank, just to inspect the seal. If there is a problem with running too lean, that might be the second, or third thing I would inspect, though.
Good idea to inspect the clutch disk bearings, though...
...With a new kit MAB, IMO, the rag joint and chain idler are the most treacherous items requiring close attention.
For the power unit, the carb will require the most attention.
And I'll voice my opinion again, since I've hopped up on the soapbox; a coaster brake has NO PLACE on a motor assisted bicycle !