Hey Mark,
I built an MK Spinks Akromaster about 15 years ago. Very similar dimensions to the Brushfire. Weighed around 8.5lbs with an ST .90K. Plenty of vertical. Could fly any FAI schedule (at the time) with ease and had fixed gear. No coupling issues! If you go back to the 1985 team trials (If I remember correctly) there were at least 2 MK spinks there with 1.20 4-stroke power. You'll be fine with the ST 90. I did try the Tiger's Paw 'muffler' (which was ST's pseudo tuned pipe - more like a Magic muffler if you remember them) and had a significant power increase. I've used an Ultrathrust muffler on an OS 91FX with great results. Food for thought.
Will B.