With an MR in that condition, the first thing I'd do is read the shop manual cover to cover, especially the carburetor section:
MR175 shop manual hereAll the problems you've mentioned - and many you haven't - have been covered at this site, some in great detail. Since the search engine no longer accepts 'mr175' as a valid input, I'd try searching my username (which will bring up a lot of threads) and then going through those for MR175-related topics. If you intend to get this bike running well again, it will be the best hour or two you ever spent.
The frame and engine numbers on 70's Honda 2-strokes almost never match when new. 1st gear is very low on the '75 and '76 MR175's.
I've had my frame welded 3 times, once at the brake-pedal pivot. If the threaded piece for the pivot bolt is still there it should be no big deal, but if it's missing maybe you could weld in an M10x1.0mm (fine thread) nut instead.
The bike came with one spark plug and a 90061-358-000 head sealing bolt installed in the other hole. This bolt is available on eBay etc., but honestly I'd concentrate on the carb first, there's surely plenty to be done there.
Ray