In my opinion,, modern fork on old geometry Bike (Jo Murray's geometrie) is an error. The modern fork are too tall !. Ok for downhill, but it's bad for climbing (the weight is more on the rear wheel). I rode a sunn titnium. Very good bike. The day, I put a marzocchi Xfly on the front, I lost a lot off foun and pleasure with it. The fork worked fine but wher I climbed on the Bike, I had to push the bike.
For your beautifull Hei Hei, you have to find :
A project 2 titanium fork (if you are lucky). A rigid titanium fork or Ritchey steel fork would be ok.
On other hand, for old suspension fork : The 1998 Sid fork (they seems to a Judy), Pace RC 38 forkâ€¦
There's a solution for your FOX : use some adjusting shim for pulling down your fork to 60mm of deflection. The fork has to be under 425-430mm tall !
A 80mm fox is around 440-450mm : It's too high !
Modern fork are good for a rear suspension bike, but, it's killing the specific spirit, and the special feeling of a titanium bike
Overall the angles looks very similar to my 97 King Kahuna with a 97 Marzocchi Z1 on the front (which is tall compared to most 100mm forks).
Ride wise is should be fine, as mine climbs with ease with no ill effects of the longer fork.
I see no issues with a modern build on an older frame as it makes them more usable with less worry about breaking things etc. If you are happy with the ride then it really doesn't matter what we think.