If you download Autodoc, they have a selection of Aerials. Just had a quick look and Beru do one for about £13, which is probably about as close as you'll get nowadays.
Unfortunately, I don't know anyone in London. But if you're transporting it and want to go further afield, I know a very good garage in Somerset. Called Hillside Garage in Chard. The chap who runs it used to be a VW master tech when the mk2 was about and he's had a few as well.
Forgive me for asking stupid questions, you have done a lot of work, but have you figured out if you have spark and fuel? Essentially, if you have spark, fuel, air, compression and timing, it should start. You've proven compression and timing, air should be a given, so I'd go for spark and fuel.
First thing I'd do is pull out a spark plug, connect it back to the lead, earth it against the chassis and test for spark. Also, inspect the plug and see if they are wet.
It's quite common for people doing engine swaps for mk3 engines to go for a widetrack setup. This is where you swap the arms, track rods and (sometimes) uprights from the mk3 unit and get a wider track. If you have put mk2 items on a widetrack setup, this will give you these symptoms.
Any reason you need to get oversized pistons? When I have done it in the past, I've found there are three sizes to piston rings, depending on the wear. I've just measured the bore size, post honing (BTW, used a dingleberry hone as it wasn't bad), fitted the corresponding piston ring sets to the stock pistons and everything worked fine.
Where abouts in the world are you from? Reason I ask is that country dependant, VW put different engines into the golfs. You mention that you have a 90hp injection engine. In that period, in the UK, the only injection engines the mk2 had were the GTI engines, both over 100bhp.
However, there are a lot of similarities, for instance, the first thing I'd check would be the coolant temperature sensor. This is (possibly) a blue sensor, usually mounted to a flange on the head where the top radiator hose is fixed. Get the car running normally, then disconnect it, if the idle changes, it's working ok, if not, then there is a problem.