This part sure does look like overkill for holding onto the metal motor clip.
I don't have the "factory" specs on the part sizes - they may differ slightly.
I pulled out one of my kits, and to the best that these old eyeballs can see:
ID is approx 0.97 to 0.98 inch
OD is approx 1.10 to 1.12
(yes, the wall thickness is heck-for-stout)
Part length is 2.10 inch
If you have the kit instructions, you know that this slips over the BT50 motor mount, with the metal "engine" hook in place so it also slips inside this retainer part. If you don't have the official Estes part you could accomplish 99 percent of the same thing by using a 2.1 inch length of plain BT50 (or heavy-wall, if you have it), slitting the BT50 lengthwise on one side, and slipping your home-made retainer over the kit MMT (part 30373) and hook (part 35021) with a good layer of glue under the retainer. Center the slit on the side of the MMT opposite from the metal hook, wrap with a gob of rubber bands, and let dry overnight.
And welcome to TRF! I hope some of this info was what you needed, otherwise come right on back and post another qstn.