Well, I'm pretty cheap myself so what I'd do is just get a double or triple chainset that works with your bottom bracket, which is pretty much anything out there since it's pretty standard, and take off the extra rings, which is what I did a while back when my front derailer broke. The chain did pop off once or twice on big bumps, so the chain guide might be a good idea.

Only problem is that you might need to get some short chainring bolts or some spacers, my bike had a swaged crank permanently attached to the chainring so it was straightforward enough. Maybe you could even put a angle grinder to your existing cranks and try to cut off the small chainrings? If they're welded they probably don't have much resell value anyway.