I think a 91 class four stroke is right for the Hog Bipe, some people report bringing the plane in at 7.5 lbs but mine came in at 9.5 lbs, so figure it will be somewhere between those two numbers depending on what you cover it with.
For me a 70 class four stroke would not be enough power even at 7.5 lbs.
At the low weight a 91 four stroke will give you unlimited vertical and at the high weight you will be able to fly through decent sized loops.
The Magnum 91 four stroke is a very nice engine that does not break the bank.
If you only operate the premium brand engines you can't match the performance of the 75AX with a four stroke without spending lots more money.