I've got the old SP 105 running now. Thanks to everyone's help, we got good spark, then I had to replace the clutch side crank seal and the rear AV mount which had no rubber at all. I replaced the intake manifold while I was there, even though the original was fine. I patched up the muffler which was completely rotted out - it's not easy to weld on this old thin metal, but I got it done.
I replaced the carb with an aftermarket adjustable one from a 390XP. The carb just needed the fuel nipple swapped from the original carb, the casting around the needles trimmed off and the choke tab turned 90° on the shaft. The shafts needed a tiny bit of tweaking, but it pretty much worked as is. I drilled a hole in the side of the airbox to get a screwdriver in to adjust it and plugged it with a rubber flange I found.
It was missing the manual oiler button, and I couldn't find any for sale, so I made some guesses and measurements and 3D printed one.
It fired up in 5 pulls on choke (didn't even need a prime!) and runs fantastic, even the automatic oiler works perfectly without any adjustment (a first for an Mac I've owned). I popped my 36" bar and .404 chain on it and went out to find some worthy wood. It'll idle all day and pulls like a train, they are such fun to run. Because it doesn't rev super high, it sort of feels slow, but it certainly isn't!
Beautiful day for picture.
