No I don't own a mercury bike, I own 2 early 650 engines, but I do talk to a lot of folks on all topics especially where I have interest.
All NHT small bolt pattern heads and engines get a lot of my attention. The parts dealers have inventory of long stem valves 1.4" like in a late 18 engine variant used in the late 650 and mercury bikes. I don't have the SN cross over in front of me.
It's some time tough researching and then describing where a bike like atlas uses a 20 engine and a 88 bike uses a 122 engine but a 650SS, DL or mercury bikes all use a 650 engine.
Counter that with Commando bike uses 20M3, 20M3S, 200000, 300000, 325000+ engines. This is without even getting into body styles LOL