My first thoughts was the screws are being overtightened causing the heads to stretch and break off after a while. The next thing to cause it is excessive vibration that can stress the screws and cause them to break. Now the OP stated the engine was in a profile plane, so right off the bat we have excessive vibration then. But with a Saito 82 I would think that a 8/32 screw would fit. But then if the mounting holes are a little too tight for 8/32 then there is a metric size a little larger than 6/32 that would work too. Also using a screw with a shoulder on it (the unthreaded portion next to the head), as the shoulder would strengthen that head end of the screw more and maybe that would help solve the problem. One can always cut the excess screw length off as needed too.