The stock transom is 25". But I agree that you absolutely should measure the transom height.
A 150 is adequate power, but nothing near generous, and will get you a nice, economical cruise in the mid-20's, and a WOT in the 36-38 mph range.
Ideal is probably a 200-225 Etec, Opti, or 4-stroke. These 21's were built to hang heavier motors than the classic 20's, so don't worry about that factor. And because the hull is longer than 20', there is no legal limit to the HP you hang off the back. You could hang a 350Sc Verado if you wanted.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury