I've read the thread... it's a great one. But the more I think about it, it would make more sense FOR ME to find a low mileage engine and go that route... IF it does need internal work.There is a great thread on the forum by ERROR85 in which he rebuilds an ST1100 engine - and as far as I can tell, it’s one of very few treads in which someone gets inside an ST engine. I guess these darned things are just so tough.
On finding the part you need - try to get the OEM Honda part number - and then just type it into your search engine (JUST the part number - no words or any other descriptors) - and you might be surprised to find that there are some around. I also restore and ride a couple of 1970s Yamaha XS650s (think Triumph / BSA / Norton - but with actual working lights) and I usually find parts by simply sticking the part number into google - and virtually in every case, something comes up. You can find the parts numbers from parts schematics at stores like Partzilla or Discount Honda Parts.
The other place are motorcycle wrecking yards - there were quite a few STs around and because of their age, they are being taken off the road - the parts are becoming available that way too.