Beefing up A500 overdrive

I have read a few articles that suggest the overdrive on a 42RH can be rebuilt to be as strong as that on a 46RH. The larger part of this seems down to fitting more clutches and steels. It seems there are different thicknesses of these but also at least 3 different designs of direct clutch drum allowing varying space for clutch components.

If it's an easy upgrade I would have expected A518 direct clutch drums to be readily available. I have seen a few after market drums advertised but none from the usual suspects - Sonnax, Transgo etc.

Is it as simple as just changing clutch drum and frictions / steels / pressure plates etc? If so, does any reputable company make a kit including the drum? Otherwise any recommendations for a decent quality drum?

If I were in the States I would probably look for an overdrive from a 46RH but it's both difficult and pricey from across the Atlantic