In theory if the injector pump is working you don't need any premix, but until you can see that it's working because the tank is getting lower as you ride it I would use maybe 100:1 as a precaution and even more on break-in, like 50:1 in the first tank.
I've run pre-mix in an RD350 and an RZ500 as well as every sled and single I've ever owned, but none had a bearing hidden behind another like your engine has. Can you put up a photo of the upped case half without the jugs on it? I'd like to see what they look like from the top. Most engines have an oil drip hole above the main bearings, but I'm guessing yours doesn't.