G'day Ukie,
If by old combat you mean the vintage variety mostly using 2.5 Diesels, I'd suggest you get a hold of one of Steve Rothwell's R250. A real gem of a motor, based on the Oliver Tiger. http://www.smrpl.com.au/_engines.htm
It's currently cleaning up in Vintage A team Racing.

A number of them were used at our nationals in Vintage Combat to good effect. ie strong enough to pull smallish 60's combat wing through very tight manouevers.

For a sport 15 I love my Thunder Tiger 15's on 10% nitro with an 8x4 prop, just nice!!