I have to agree they are pretty easy to work on. Couple years ago I completely dis-assembled one, cleaned it up and painted (my first so wasn't great) and then sold it. Good thing I had 2 at the time and could refer to the other one when reassembling. But no better way to see how they go together. Built like tanks, 99% metal parts, all the workings out in the open to work on. Just great machines. I'm always looking for the old self-propelleds too although cant buy every one I find. Hav been lucky enough to get two really good ones free though (the 21401P from original owner and the 2cycle WR). There is one for sale about 25 miles from me that is not sp but looks in near mint shape for $80. It's mid 80s with Tecumseh engine, the one with the big air cleaner in front with the tube that wraps around to carb in back. Don't really want to spend $80 on non-sp but it's so clean. Maybe he'll take $50.
That's cool u chronicled your work. Look forward to those pics.
. i have several ive restored. i restored my 1997 moldel ninja self propelled that i bought right after my mother passed away, just finished a 2003 ninja that i got for $25 and was gonna use for parts but was in way to nice shape to part. now i have the 1980 that i bought last year used for my father before he finally this year bought a new hi-vac. we have been using snappers for 30+ years and have always had good luck with them. they are the best built and very simple to work on. i buy up all the used self propelleds i can find. now im lookin for an aluminum deck one and also a wisconsin robin like u have. 
