Welcome to the site and thanks for posting.
Manx GP has always been on my own bucket-list but I pretend to know nothing about the formal side of the racing.
I had a quick look for race association for the Manx GP but only came up with the following: http://www.manxgrandprix.org/MGP/MGP-Riders-Association
I'm sure someone more knowledgeable will be along shortly.
Its quite odd to me that I'd only just been looking at my photos from a ride trip to BC from 2015 and hoping I'd be able to come back one day and here is you making the reverse trip!
Hope it works out well for you.