I did 3 forks yesterday, including two Z1s, which was my first go at it.
It's not too hard, the main issues I've had are removing the brace as the bolts can be hard very tight, although it can be done with the brace in place. It can also be very messy.
The best video to watch is from butter suspension on YouTube, just search "Butter Suspension Z1"
I found the most useful tool is a Pedros DH tyre lever and a normal plastic tyre lever to remove the seals, this was recommended in the video. For the oil I used Sikolene 7.5wt fork oil that I got from a Motorbike store.
If you do want to get it serviced by a professional I think Pace Cycles still service older forks.