Buddy rides a Klein Aeolus Tri bike, 1994 and while attempting to replace headeset, noticed dual cracks across the back of the fork.
The cracks appear to be where the fork blades were bonded to the fork crown and do not appear to separate when pressure is applied. I believe the fork is aluminum, it's certainly not carbon at that vintage and as it's the stock fork.
Still, I wouldn't be riding this bike.
So he needs a new fork. Trouble is it's [EDIT 650C] and 1" threaded, so can be 1" threadless as well, but who makes such an animal, besides WoundUp, which is way out of price range.
My general opinion is New Bike as he's long overdue, but told him I'd do what I can...
Keep searching. I think you should be able to find a 1" 650 fork. I do not believe there is any difference between B and C, those are rim spec dimensions. What is important is the dropout spacing, probably 100mm, the crown to axle length, and the rake offset. The length and rake offset will effect steering trail but the replacement fork does not need to be exactly the same as the original, just try to be close.
Of course the 650 mm is the overall approximate wheel diameter including the tire.
I have certainly heard that Aluminum forks fail although I have never seen one fail (I currently have one and those pictures give me the willies). I would say that the fork is certainly done for and should not be ridden. I don't really know if there is any feasible way to weld the fork but being aluminum I would say no.
I have heard that 26 inch forks should work with that. I mean there is always a chance of jimmying the wheel into a different fork but I have no experience with that and don't want to lead you wrong there. But perhaps finding a fork with a similar sizing to the wheel and then either dealing with some long-reach calipers or the like would help. Might want to check an online parts site like Nashbar or Performance.