Yeah the 120 4 cycle is a great choice. The OS 120 4 cycle is 33.3 oz, and setting up everything by the manual I added a 1 oz gold spinner hub. So I will say if you are going to go by the book get a engine that is no more than 34 oz. If you go with a larger engine you will need to move the servo/ battery tray around so that you will not have to add any dead weight to the tail. Also the Gee bee lands best with a 6 pitch prop, because 8 inch prop does not allow the gee bee slow done on the landings.

There in someone here with a super tigre 2300 (1.40 , 38 oz) engine in theirs and they said it flew great. The OS 1.60 is 32 oz w/o the muffler and with a bisson muffler it will come out to about 36 or 37 oz.

The OS 1.60 you could swing a 19x6 just fine. and the super tigre you could swing a 17x6 or 18x6.

The 19x6 will probably not give you good clearance, a 18x6 is the most that I would put on there.