Nema: JvdZ' build, Holland - Builds - FOIL.zone
Modified Nema23: Nibblers Build from Germany - #5 by Nibbler - Builds - FOIL.zone
Cooling of china gear: Daniels (un)geared inflatable (slowly built) - Builds - FOIL.zone
The 2 alu parts on picture 3 that fit around the gear, you could just use a ring or two ring segments instead
Cooling of motor is just through contact with the tube id 56mm.
If you haven’t started I suggest to use a direct drive, either 65151, 120KV: Flipsky 65161 RC Car Brushless Motor Sale - Banggood USA
or 63100 run wet: https://alienpowersystem.com/shop/brushless-motors/63mm/aps-63100-outrunner-brushless-motor-140kv-4500w/
No gear means a shorter pod, no hassle with couplers and water proofing, much easier