The stats I think they should have are:
attack 2 (I think the PT boats should be able to bombard small islands, islands contained within one SZ, hitting on a 1)
defense 0 (all though to account for speed after enemy rolls if they hit the PT boat my roll, if they hit on a 1 then they can avoid the attack, also I think on the first round of fire each PT boat may fire one anti-aircraft volley)
They should not be as powerful in there attack as a Sub. Unless its an attack of 2 in the 1st cycle only. Bombardment from such a cheap unit is too much i think. you could spend 20 and get 2 PT boats, A Transport, 1 Inf., 1 Art. and make an amp assault getting 2 bombardments. AA would also be Too much… you could potentially have cheap AA guns in a bunch of SZ’s.
You can’t always represent Actual combat abilities for every unit on the game board, because the game generalizes the various units. for example 1 inf. probably represents a Squad or company of troops, but a fighter unit may represent 3 fighters. etc.