Ant, it's really simple. You are blowing your motors due to bad tuning if you are going lean and cooking pistons. Get a standalone like I have, rebuild the motor with some forged pistons. Also, we need to determine exactly what happened when the motor lost compression. No cracks in the block, correct? Just pistons and rings?
PM me, and I will get you a top level tuner to come here and tune your car correctly. I can help you get your car ready with checking all of the pre-tuning items, like BSFC, etc.