The drive shaft splines tended to fret use a good spline lubricant and check the seal surface on the drive shaft the steel rusted and cut if so a speedy sleeve does a good job before replacing the seals in the w/p base
I remember a few of those 650s from about the 1970 era that idled very well…. even without a thermostat. Like all pre-1976ish Mercs, water in the gear case is almost a given. Ideally you will want to rebuild and reseal but at the very least, refill it with oil and spin it over with the starter to preserve what may be left.
Many 650 / Panther Merc’s (OEM) have the Thunderbolt Ignition, notorious for breaking the timing belt…a "fix" is to substitute a Kiekhaefer 4-D magneto, as it has less drag.
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