Yes, you can replace your cluster with one from a Z28/IROC. Some Z's &
IROCs had 145mph speedos; most didn't. They turn up occasionally on
E-bay. Expect to pay too much there. Hit a few junkyards & maybe you'll
get lucky. The 145mph cluster was available from around '86 to like '89
maybe? I think 'bigfig' might have a better idea on the years.
Unless your car is modified, I wouldn't bother though. My '89 RS is
speed-limited to 115mph (305TBI-5spd) so the diff. speedo would be kind of
pointless, unless you are just into looks.