Welcome to Compact Jr. ownership and to this forum.
The original vinyl collar was bolted and glued in place. The collar should be attached to the inside of the upper, raised portion of the roof and to the outside of the lower, fixed roof. When I installed mine, I ran a bead of Liquid Nails panelling adhesive between the fiberglass and the vinyl. If there is a tight fit on the collar, there is little need for adhesive but I used it anyway. As Frederick said, I don't think it would leak water without glue.
on my Compact Jr. are Goodyear Marathons ST 175/80 R 13 and I run 35 lbs. of air in them. No problems in 3 years of use.
I know of no source for parts other than generic RV suppliers. Most parts are generic to all trailers of the 1970's.