OK I've gone brushless, what a difference!
My solution was to fit a small direct dive brushless, a KDA22-10s. I filled the hole in the foreward bulkhead where the gearbox used to sit, and mounted the motor in front, cut out the cowling to fit over the motor. A supplied adapter is bolted to the front of the motor and the standard prop hob drilled slightly to bolt up, also using standard prop blades. Looks a little weird as the prop now sits further forward from the cowling, but this may allow some airflow to motor.

Gave good rpm on bench with two cell lipo, but have flown with 1300 mah 3s lipo. I am pleasantly surprised with this little motor and my much rejuvenated Sky Runner. Will fly vertical.