To start with 3 HP:
In PRINCE.EXE, search for C7 06 08 46 02 00 and change 02 to 03.
(Alternatively, you could remove the loose floor from room 5. Then the gate at the start won't close and you will have time to drink the big potion.)
By the way, the trick at the beginning of level 1 is hard to do even with 3 HP.
It seems that the exit of level 12 was moved to room 24 instead of room 23. (Room 24 is in the pit on the right side of the starting room.)
In PRINCE.EXE, search for 80 3E 61 43 18 75 0F and change 18 to 17.