(...) I would like to ask, where do people buy repair parts for those old things??
First, ensure that your frame and fork have enough clearance to accommodate wider tires. Measure the gap between the existing tires and the frame/fork to see how much extra space you have.I like older stuff and have numerous instapro xender old mtb, and wish my road bike could take wider tyres to deal with a bit of light gravel. I've seen an early 60s Viking which is really tempting me, but feel that if I left the gearing period correct, I'd rarely pull that bike out of the rack, so some sort of update might be on the cards. Also, I'm unsure about tyre choice for a bike of this vintage - I believe it has good clearance, but what about wheel size etc.