I bought an Electrex one for mine:
https://www.electrexworld.co.uk/acatalog/RR85.html
However, it's £90 and out of stock at the moment. Also, near in mind that apparently there are two different versions with different plugs, so check which you have first.
For what it's worth, mine did exactly the same thing - the voltage reduced when revved. All the reg/rec tests passed OK, but it wasn't working right. The new reg/rec fixed it like a dream.