I just replaced the 105 crankset that was on my 520 with a Sugino XD-600 46-36-26. I've seen these advertised on e-Bay with several different chainring combo's, all for about the same price (about $80). Best yet, they utilize a square tapered bottom bracket which is what your '92 Trek probably has on it, unless you've upgraded more recently. Happy Riding!
CC