An old singer sowing machine (Silverdale LA5)

The machine was working fine, but when I changed to a different colour thread, it didn’t sow a seam anymore and if only in irregular length Stiches. I tried my best to solve it by fiddling about, but couldn’t solve the problem. It is probably minor and some with more experience can fix it! That’s my hope as it would be a shame to give to landfill because of that. Also one side of the cover holder has come off, therefore the belt …