I have a project bike in the works, and looking to hit about 70 gear inches on it. I got a flip-flop rear wheel for free, which has 16t cogs on both sides. The stock crankset is an old steel road double with a 38t inner chainring. It looks like a 42t chainring would get me into the territory I want, but I don't see much in the way of 42t single cranks on the market, and I'm guessing I'd need to replace the BB (or at least the spindle - I don't think it's a cartridge on this bike) if I did the crank. Chainline looks good with the inner chainring, but it's too small to be of much use unless I can find a 14t freewheel (does that exist?)

I'm looking to put as little money into this bike as reasonable, while still getting the desired functionality out of it.