If you want to go to Hollowtech II and outboard bearings, Phil Wood make a pressed in outboard bearing set up for Kleins.

Phil Wood & Co.

Phil Wood also sells Klein compatible spindles in a variety of lengths and both ISO and JIS tapers. (Even Ti spindles.) There is nothing wrong with a quality square taper crank.


While they don't explicitly state this will work, chances are it will because the Klein shell inner diameter is 35mm, just like the French shells it is designed to work in. It is self expanding and offers a variety of JIS spindle lengths.

https://velo-orange.com/collections/...ottom-brackets


The Klein system is simple, offers control of spindle centering and is very tough. It is also pretty cheap to replace just the bearings. It is not difficult to perform removal and installation with ad hoc wood blocks, crank bolts and a mallet. The old bearings can also serve to press in the new ones.