Is there washout in the wing-tips? If so, & if the wing was weakened torsionally in the initial crash, the tips may be washing out more with increasing speed, causing the tuck-under.

I encountered this on a student's LT-40 -- normally a docile airplane. He had a crash involving a low-speed cartwheel in which the elastic-retained wings popped off. No visible damage. On the next flight, the plane lifted off & climbed out just fine, but when it levelled off and began to accelerate, it tucked under. Closing the throttle & applying full up elevator eventually resulted in recovery, which was abrupt. Many fixes were tried with no success, until I was handling the wings one time & I noticed that they seemed to be more torsionally flexible than I thought was normal. On speculation, the wings were stripped, joints re-glued & the wings re-covered. Voila!! Problem gone.