I have a 1993 RC200 I would like to get sorted and use it again. It has a 1" headset, about five years ago I bought a pair of 1" RC31 forks when they came out.
It currently has a 1" syncros aheadset and a 1" Syncros cattleprod aheadstem. The plastic top cap will not withstand the torque required to tension the headset properly, and never has. The headset / stem also works itself loose slightly when I ride the bike
Is it possible to either get an alloy topcap for the syncros stem (unlikely) or a shim to allow the use of a 1 1/8" aheadstem?
Yes, you can get a shim to 1 1/8 very cheaply, but I wouldn't give up on that classic Syncros.
Firstly, I'm sure I've got a one-inch alloy top cap from my lbs in the past, but even if that's not possible these days, why don't you just use a big washer to tension the headset, then tighten the stem bolt good and proper (as you would anyway) and then replace the washer with the plastic top cap, which you wouldn't then need to torque very hard?
If the stem bolt isn't holding though, you do have a problem with the stem - it's that bolt that's supposed to be keeping everything tight - not the top cap.