I've mentioned my Nishiki Semi-Pro here before, below are some pics. I bought it, new, in 1973 or '74 in NYC for $150. It was billed as light weight at 25 lbs at the time. There's a fair amount of rust for a bike that was never left outside, but in the garage. I attribute that to New England humidity. When the barend shifters finally ended all pretense of friction and the rear derailleur's penchant for skipping the middle gear (of 5 originally, then 6 cogs) on downshifts became an outright compulsion, I turned it into a fixed gear using the original Sugino Mighty Comp crank's small 48 tooth chainring with an 18 on the rear. The only new equipment was the rear cog, everything else stayed as original including the cable housing and brake pads (it had its tires, chain and brake cable replaced over the years of course). The only mod during the conversion was getting rid of the brake levers you could actuate while riding upright on the handle bars, putting new hoods on and using 3mm spacers between the crank spyder and chainring to get the chainline exact.

I really didn't like riding the fixed setup so gave a single speed a try, again with an 18t cog, and now it's really a pleasure. It's a very simple process to convert, costs almost nothing, and makes the old technology enjoyable again.