Only a random thought but are the pistons in the right way around. Reed holes to the exhaust does 'bad' things. Also it was common on the early RD's to get the slides into the wrong carb which put the cutaway forward. Wouldn't stop it running but ran rich like the choke was on. Center crank seal good, by memory they were only an alloy grooved seal but rely on main bearing being good so as not to touch & oversize the close fit of the seal. Word labirynth seal comes to mind. Good system realy as in theory nothing t wear out.