I've just won a Railpower SD7/9 loco kit with an Athearn chassis. What can I use for a donor please.

Not sure what you mean by a donor. The Athearn chassis, if powered would be the donor. Do you mean detail parts for it?

Basically , yes. Glazing, handrails etc are needed. I now have a set of Hitower trucks and suitable sideframes. Any other parts I'd need to keep looking for. Mechanically I'm sorted I think.