1. Buy a masonary wheel and attach it to a circular saw. Score two lines about 12" apart from the drain line and cut a 1/4" groove. Rent a electric jack hammer and break and remove the concrete. ( there may be wire or rebar that you will need to saw off.} Remove enough soil to relocate the drain line to the correct distance. then refill and repour cement.
2. Contact a spa / whirlpool fiberglass Co. and have them fabricate a tub with the drain hole at your required distance