I sold you the bike. Love to see some pics when you go for the colour change.
I think you have to go with the forks you can find. As a 1991/2 (Flare red with taller and narrower seat clamp) you probably need the later Uni-Klein rigid fork with rounded "shoulders". Both the Uni-Klein and box are rare though but a few have popped up on eBay this year. You will need the exact measurement for your size frame measured between the bearing interfaces. In the meantime get some elastomers and ride her with the Manitous as you will probably get £100 tops for these. They are more interesting with the Klein steerer though.
Not disagreeing with the above suggestion however as there are many with Box and many with Uni-Klein from 1991/92 but the Flare red appears in the 1992 colour chart so it may suggest it is late 1991 or early 1992. I don't think anyone will argue with you as the new colour will dictate the right choice. If you go with a Dolomite for instance then a Box will be better and go with a HLF and Uni-Klein etc..
Thanks