I'm not sure about the availability of a "factory kit" but boxer2valve sells, what I believe is, an aftermarket 5 speed shift kit that fits the 74 R90S. There are several photos on the product page that shows the difference from the original. I too have a 74 and will sometimes find neutral between 4th and 5th. What does seem to help is being positive with your shifts. If you(I), for lack of a better term, soft shift, it seems to happen more often.
I hope this helps,
Scott.
Scott Blaylock
Stoney Peak BMW Service
Victoria, BC. Canada
BMW Classic and Vintage Motorcycle Service and Restoration service@stoneypeak.com