Upgrading the forks on a 1995 Diamondback Vertex

I have a Vertex from around 1995 and it has a pair of manitou forks
on. They're prettymuch knackered but I really like the bike and I
can;'t afford to replace it anyway, so wanted to change the forks
out.
Anyone know what I need to measure to find a pair that will fit? I
know front shocks have chnaged a hell of a lot over the last 10 years
so does anyone kmnow what I need to look out for in a new pair to make
sure they fit / work etc etc? I'm looking at buying 2nd hand so can't
rely on help from a retailer.

Thanks in advance for any tips

John

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