You may want to do a little more research. The way I read it Aero Shell Sport Plus 4 is the only oil currently recommended by Rotax. Mobil 1 covers a lot of oils. The one that was previously deemed as acceptable was Mobil 1 4T which is motorcycle oil. It has the additives necessary for the Rotax gearbox, But is not suitable if you are running 100LL. http://docusearch.flyrotax.com/files/pdf/d06038.pdf
Actually those hoses are the hot water coming from the heads. The 1" hose is the outlet and that should be routed to the rad. The 1" hose coming from the other end of the radiator then goes to the water pump which Is pumped out to the bottom of the heads thru the heads and back to the expansion tank. So now hopefully my last post will make more sense the tee's are then 1" x 5/8". So if you want to use copper a 1x1x1/2" sweat tee will work for a cheap solution but does not allow much clamping room. The wye's from UPI are more expensive but were worth it to me. UPI's part # for the Y is Z17550 it is a little hard to find without that. https://www.universalpartsinc.com/
Tee to the core after the expansion tank before the radiator and tee back in after the radiator before the 1" returns to the pump. I used y's facing to optimism flow directions but I know several others using tees that work just fine. The core is connected parallel to the radiator if that make more sense.
I had the factory heat muff on the muffler and after many attempts with no noticeable heat went to the hot water heat on my 912. Works great now other than all the drafts I have not address. Built my own for about $150. Fifteen dollar heater core from NAPA $60 fan from UPI. Guess I have less than a $100 into it The middle px is of the shroud before the fan was attached
What type of resin are you planning to use? I would suggest Epoxy because it is mush easier to sand. Polyester resin is a bitch to sand, plugs up the sand paper. With Epoxy you use micro balloons mixed with the resin to fair it up which there will be a bunch of that to do. I like the West system Epoxy because the pumps make it easy to get the ratios right. Anyway just my thoughts. I have built a 24' sailboat with polyester a Q-2 with Epoxy and two cowls with Epoxy and would not go back to polyester for anything. Vinyl ester may be an option that I would consider and then use Epoxy with micro to fair it up. what ever you decide to use a small tent with a secretary heater under it over night will make the cure a snap. The Rutan glass is great stuff to work with, wish I was there to help, it is very satisfying to build a cowling.
I had one of my semi' drop a driveline, lost the nut off the input shaft and was able to repair well cobble it together with some 1/4" nylon rope and drive it from Bozeman MT to Casper but that is a story for another place!!!
That Epoxy primer that you are referring to is for metal. So you will need polybrush and polyspray for the fabric, do not use the Epoxy primer on the fabric. The Epoxy primer is also used on fiberglass.
The earlier Grove gear can sometimes be straightened but they have been using 2024 for quite a while now and they will not straighten those. Best bet is to give them a call with the serial number and they will tell if it is possible.