Chromatic repair

I have a 280 Chromonica which I took apart to clean. I can't seem to get the mouth plate back on tightly. I had Hohner send me some new clear bushings that go into the comb that the screws screw into - but they just don't tighten. The screws just spin and spin, but never tighten which leaves the mouth plate, slide and comb cover loose. There appears to be no damge to the comb or the screws. Hohner wants $150 dollars for an overhaul since it is no longer under warranty and I am not willing to spend that kind of money as everything else on the harp is fine. Any ideas on how to fix this problem? Thanks!