Neal Stepp at ISS is a good source for parts. Best to call him at 817-595-2090.
He will ask you to describe the problem and then send you the parts. I redid a 600 a few years ago and the postage was more than the parts. It was easier than I expected.
If you haven’t bought one yet, get ahold of Carel in the Netherlands. He sells on AA, TargetTalk and others. Stand up seller who comes up with some very nice FWB’s.