A short travel fork such as a Judy would be a good option. 100mm would be too much for a Catamount.
There's no real answer to which fork is better, they are both good forks and very different.
As for parts availability, I would think a period correct Judy would be easier to maintain, as it's a simpler design. Which ever you do go for, buy the best one you can.
Be nice to see which model frame you've got.
al.