probably unnecessary to buy a new crank, though if you have the cash I'd do it because a newer crankset is probably lighter. I would say something between 107 and 110 will put you in the right ballpark for your rear spacing, pending some clever work with washers/spacers/flipping chainrings etc. Maybe look at the old bottom bracket spindle and see if there is a number in the 100's marked (probably 105-115ish). That should be the spindle size you need.

Edit: take some pictures and we can probably help better.