OP,
I bought a '92 Attitude that had been retrofitted w/ a Judy FSX which, although really cool and fairly rare, made the bike handle like absolute crap. I rode it on a fairly tight, twisty trail and the fork jacked up the geometry enough to make the handling skitterish and vague.
Keep in mind that my Judy has only a few inches of travel as opposed to the newer, higher-travel forks...
Luckily, the seller had the original rigid fork and i had that swapped in and it made a huge different in the handling.
People are telling you to sell because Kleins of that vintage are worth pretty good money and those of us in the vintage game like to see nice vintage bikes being treated with respect. What you're asking is like taking a '67 Vette and adding parts made for a Honda Civic. Sure, they may ARGUABLY make it perform better but that sure as hell doesn't mean it's a good idea.
But, it's your bike so if you REALLY want to add a suspension fork you'll have to find an adapter w/ the proper sized steerer. I'm sure they exist but I haven't seen any others besides mine...and no, you can't have it
EDIT: the only caveat to my advice is if the frame is in crappy, but structurally sound condition. In that case, a "resto-mod" is a bit more forgivable.