Replacement hose for coolant overflow bottle?

The 912 i Series Installation Manual indicates in Figure 8.6 that the hose connections are 8mm. (It's a lot easier to find this info in the install manual for the carburetted models, where they put it in a table along with the sizes of all the other coolant hoses.)

This hose is not supplied by Rotax, so if your aircraft is factory produced (S-LSA, TC'd) it's probably safest to get it from the local sales agent for your aircraft, where you can have some assurance that it will be legal to fit to your aircraft. For experimental/amatuer built aircraft, I personally would suggest looking at using automotive heater hose, (SAE 20R3) which should be easily available from most automotive suppliers.

Thanks, Tim! Just bought this silicon heater hose and hoping it works! https://www.amazon.com/High-Performance-Silicone-Heater-Hose/dp/B0D1J68WTY/