Dont have any info to hand, but Ive done this when building mk2 20v's and the ECUs havent bitched about lack of speed signal (many many other things, sure).
Googling found a bunch of info, including this from rubjonny on clubgti, I cannot confirm this, but he is usually right:
you'll need the speed signal for the ecu, to do this in a mk1 you'll need an MFA cluster and add a wire into the smaller plug on back of the dash:
T6
1 - wht/blu - outside air temp sensor
2 - wht/red - outside air temp sensor
3 - grn/blk - Fusebox pin B/15 on early models, spade to engine loom behind fusebox on late models (oil temp sensor)
4 - blk/yel - Fusebox pin A/3 (Gearshift control relay pin 3/L)
5 - wht/grn - Fusebox pin B/18 (Gearshift control relay pin 5/G)
6 - wht/yel - speedo signal
you may be able to add the speed sender to your existing clocks, its screwed into the back of the speedo on the MFA cluster and the mk2 golf unit will fit the mk1 speedo. then you'll probably have to find the 6 pin plug housing and screw it into the back of the cluster with the pcb section inserted, assuming this is there on the non-mfa clocks?
See here:
http://www.clubgti.com/archive/index.php?t-267196.html