Agree about the photos. Also, need wing area and weight. You said built for speed, so thin airfoil? High wing loading and thin wing can spell a lot of trouble.

The person who built this really could not have known what he was doing. As above, a CL plane should have right thrust and be set up 0-0-0. The fixed rudder is usually canted right for CL. I'm not sure how much effort I'd put into a poorly built, poorly set up plane, but that's your call.

But part of this is easy...the incidence difference between the wing and the stab is what causes the need for so much down trim. There should be no difference or very little difference. Also, it is likely you need more vertical fin/rudder area.

Most CL types modified to RC have the fuselage lengthened, but then consider the Sig Wonder or many of the RC combat planes...very much like CL types. People fly stop signs, witches on broomsticks, and lawn mowers, so almost anything is possible. But figuring it out from a description is probably not going to happen.

On the plus side, it can be a learning experience to mess with a problem airplane, and fun if you like that sort of thing.

Jim