Bicycling

If the shifters are friction, you can probably just swap the crank, though it might not shift perfectly. A new bottom bracket would help line things up better, and the front derailleur may have to be swapped to prevent chain drop.

If you have indexed shifters, you'll need to change out the front shifter from double to triple as well.

If it's only 5 cogs in the back, not much you can do beyond 14-28. Get a triple with a 28 or smaller chainring up front and you should have enough range.