Your best bet is probably to look for a biopace triple crankset on ebay. You could run your outer ring on the middle position and the small ring in its proper place if you want a 48/28 double or similar, or you could run the triple and get a slightly longer spindle. As is the case with conversions like this, you'll need to think about bottom bracket spindle length to manage the chain line, chain length, and a long cage rear derailleur.