Ron, certainly a Mk-20 power head would fit in with no issues. You would have to replace the carburetor as well, as the Mk-20 uses an AJ-41A carburetor and is jetted for 20cid. The Mk-25 power head would fit also, and it has the added benefit of the fuel pump boss already cast into the side of the crankcase. You would have to modify the lower bearing carrier on the Mk-25 power head by removing the boss that supports the throttle linkage. This boss is in the way on the Mk-25. The other advantage of the Mk-25 is that it is a heavier duty power head with a larger crankshaft and rods. The magneto would transfer to either of the blocks, but you would need a Mk-25 flywheel. (Shameless plug! If you went that way, I would really like to get your Mk-15 flywheel and point cam) (I have a complete 1955 Mk-25 power head available)

As for using a Mk-28 or larger power head, The bolt pattern will fit, but it is not a bolt on and go. Here is a video of a project I did. https://www.youtube.com/watch?v=U8wVKrm3QiI This is a 22 cid power head on a Mk-25 tower and a Mk-28 top cowl. I called it a Mark 28.5 or project 50:1 as there are parts from over 50 motors in this from 1952-72. Some would call it a Frankenmerc!) You would have to lengthen the drive shaft and the cowls would not mount. I used a 1964 200 lower cowl and modified the Mk-25 tower to fit. I don’t know if you could ever make it look like a Mk-15 in the process. Interesting idea.

Steve

How do you get the Mark 15 recoil to pair up with the Mark 25 flywheel? Are the Magna Pull ratchet nut of the Mark 15 and the Mark 25 hex nut the same thread? I am too lazy to go down stairs and look…..(-:

I would like to add my personal opinion that, if there is any way to save it as a Mark 15, you should. There are not many proper Mark 15s out there. If you just like the look, there are a ga-jillion Mark 25s, which are almost the same in appearance….. Just sayin’.