I was lucky enough to have a talented engineer make me a shimming plate before he retired. It's a lovely thing, surface grind to give a perfect flat finish.
I'm not a pro. Bob (voyager) and myself rebuilt a considerable amount if gearboxes for the uk guys. All unpaid work with just the cost of parts being paid for. I've done quite a few paid jobs as well but no longer have the time since setting a business up.