I know I'm digging this one out of the vaults, but this seems to be the most comprehensive thread on the subject and I thought I'd add some numbers... maybe this is worth adding to the FAQ even?
Just finished pulling the A/C compressor out of the mix on an otherwise stock 1.6n/a mk1. This was a forced issue after one of the a/c compressor pivot bolts snapped off in the bracket. I've been wanting to ditch it properly for years, but of course don't have the parts handy when the bolt broke, and need the car back on the road asap.
Picked up the double pulley that is referenced here, easy-peasy swap with the stock alt pulley. As stated, the new pulley does NOT have a woodruff key, so hopefully it won't slip. With the new pulley on the stock a/c alt, mounted in the stock position on the stock bracket, the belt grooves lined up perfectly and the stock alt tensioner reaches fine. After many, many trips to NAPA, the belt you need for this swap on a mk1 is NAPA XL 25-9455 (made by Gates). I had originally bought the belt that Toywagen sells with his pulley (17415... cross references to Napa 25-9415), which was much too small. 25-9451 *almost* fits, but not. 25-9455 fits perfectly--it's at the far end of the stock tensioner when installed (in fact I had to slip the belt into the groove before putting the pivot bolt in the alt) so there's plenty of room to take up wear. I don't know why the toywagen belt wouldn't fit, unless his pulley happens to be much smaller.
Hope it works. Maybe one day I'll get the "right" brackets and alt and swap it during a timing belt change, but for anyone looking for a quick easy way to solve the issue on an a/c mk1, you can be back on the road quickly this way. The a/c bracketry is bolted in under the IP, so swapping them is a lot more involved.
Luke