I suspect that the float coating has turned black and is coming off and clogging the carb. You can squirt some fresh fuel mix straight into the Venturi of the carb and if it will run by bypassing the carb then you can pretty much bet that the carb is dirty, no matter how well you clean the carb if you don’t get a good float installed the old float coating will continue to peel off and clog the carb. I tinkered with mine a bit before the restore and that’s what I learned. I would suspect the same with yours. FWIW internally that machine was the cleanest that I had ever seen, no wear at all, so I just cleaned and installed new gaskets and seals and reset the Hobbs meter on that engine. The carb, of course, is always a different story, new float and primer assy almost always along with new needle and seat.
It’s got an old school magneto with points and condenser, so you may need to go in and clean and inspect. The plug for that one is a CJ-14, gapped at .025. I’ll post a link to the manual when I get a chance, on the road now.
You may just put it up for sale as is and get decent money, but the collectors unlike me who prefer not to restore will probably not pay near as much. Heck, I never cared whether they would run since I was usually doing a 100% restore anyway.