the port is prolly to drastic to turn into a 4/7mm. id pull yours down, see whats up and go from there.
if you wanted to stroke it, youd need cylinders, crank, pistons , domes and gaskets (and a port job) (and with a 7mm crank, you need to trench the cases) seems a bit too much, if alll you need is a crank and bearings... hopefully.
strip it down, see where ya at.