I would suggest the Magnum 91 XLS, it is cheap, powerful and almost the same weight as the 75`s.
I do own the Tower 75, OS 91 FX AND the Magnum 91 XLS and the most bang for the buck is no doubt the Magnum.
Alot more power than the Tower 75, lighter than the 91 FX and half the price of the FX but still runs as good if not better than the FX.

Here`s a tip along the way, the Tower 75 MUFFLER fits both the Magnum and the OS with just a little enlarging of the bolt holes in the muffler. It will add some serious power AND save you about two oz in weight. My Magnum 91 with tower 75 muffler weighs only 687 grams ( 10 grams more than the Tower 75 engine)