Forget the Chrysler. If you buy the Merc make sure the wiring is good and the ignition is working. Plenty spend for those Merc Ignition parts. The OMC would be my choice if it is a 78 or newer.
Get the 89 20 hp. It's newer and has a better lower unit on it. It will also be a lot more powerful with better parts availability going forward. As far as the 72 model are you willing to take a small arsenal of tools to fix the magneto if it fails. A CDI failure is a 10 minute fix If you carry...
As far as not reaching top Rpm make sure the carb is opening all the way. Those rollers and arm assembly are not the most durable. How do you know its only reaching 3400 rpm.
Put it all back together. Then run it up to temp and spray Ring free, engine tune or something similar through the engine. But you can look straight up the exhaust tuner with the lower unit off.
Spraying oil in the cylinders and testing compression right away is a false test. If you spray the cylinders with oil wait overnite then do the compression test.
For anyone wondering what will make grown men lose their mind, or want to see the "figment" here it is.
https://www.sharrowmarine.com/?gclid=Cj0KCQjwhqaVBhCxARIsAHK1tiP_4icJHU3yETaGA-cOjR8Nb4Dsc7qjAJUiSSH1YPnwX0Aq6MduWUoaAqVeEALw_wcB
