Phoenix Super Decathlon? Much better starting at 90 then moving back. I think my sweet spot was about 93mm.

Like others have said above, move something toward the front of the plane, effectively moving the CG closer to the front edge of the wing. Can move the battery forward, the engine forward if need be...

Last thing you want to do is add any weight. That plane flies pretty nice (again assuming it's the Decathlon) but I flew it with an added smoke system and the extra weight took all the grace out of the plane.