I have a GP Big Stick 60 that started out with a ST 75 and had great sport flying cababilities. I changed the motor to a ST G90 / OS #8 plug / Mac's lightweight tuned muffler/ Zinger 13x6 prop./ 5% fuel and, WOW, unlimited vertical.

The carb has great transition, this combination is a blast. You certainlly won't regret putting a ST 90 in your model, plus these ringed versions last forever.

I have found that those big ST motors prefer 0- 10% max. nitro fuel to run really well and keep cool.