Stator issues seem to be a common occurrence in many bikes not just the BMW. I've been reading many posts ( not on this site ) about using series voltage regulators instead of the shunt type. I won't go into explaining the difference, as you can find that out, but it is safe to say that the series units are much easier on the stators as they just allow the current to flow that the system is calling for instead of the shunt type that shorts the excess to ground which generates heat in the stators. Most if not all our bikes are equipped with the shunt type.
From what I've read if you're stator system is a 3 phase ( like Harley, Suzuki DL650 or many other bikes ) and the total stator output is less than 35 amp something like a Polaris Regulator Part # 4012941 might work for you. Also Compufire # 55401 has been used with success.
Just another option and might be worth checking out. The above are not a plug and play as you likely will have to change the connector plugs or use new ones.
1971 BMW R75 project,
2013 Suzuki DL650A