I don't know of any "retro" MTB events as such, but there's a few guys on this forum that are from the area maybe we could arrange a mini-meet-up sometime?
You might want to check out the "Rock & Road" bike festival in Tomintoul on the 12-14th June....
The Macretro part of the forum puts on regular rides around Scotland, the last one I went to was in Ballater, and there's rides planned for Aviemore and Laggan in the next couple of months I think.
Head over to the "Macretro" Area Group part of the forum and say "Hi", or "Fit Like" if you prefer