I don't know about airleaks, the carb has an o-ring towards the crankcase and the backplate has an o-ring too. There is an oil channel to the front bearing, but that wouldn't make it run too rich I think. The front bearing on my GMS .25 also has dual rubber seals, I'm tempted to remove the rear one.

The liner is a little loose in the crankcase, but I cannot see any leaks on my engine. There is only very little material between the transfer ports in the crankcase and the exhaust port though, so there could be loss of crankcase pressure there.

There is however a real potential problem the way it is delivered with the remote needle setup. There is a plastic plug that sits in the threaded hole where the needle would be in the carb. This plastic plug is too long, and it will obstruct the fuel flow by blocking the access from the fuel nipple. It was a little too difficult to show that in an image, but I've indicated in the picture below how much one will need to shorten plug. With the needle directly on the carb this shouldn't be a problem though.