looking at picture 2 i would say possibly your fingers want closing the part where the blade slides should just clear the blade also the blade keeps could be out of adjustment that could be why it is jumping and not cutting properly
put either nuts and bolts or clean gravel in the tank and shake it about when all rust is removed it might be worth treating the tank with a sealer in case there are pin holes in it
tyres and tubes are easily available but not original but very close 2.75in x 21in mostly motor bike tyres I will find address for my supplier in North East UK
Hi Charlie
My copy arrived today, renewed membership online and also downloaded and printed the Calendar great idea Merry Christmas and a Happy New Year to everyone Dave