Aside form attacking the air leak, reread you engine manual or instruction sheet to verify what glow plog you should be using. If the instructions specify to use a short glow plug, use the short ones only. The long plugs will significantly increase the compression ratios on some engines such that the engine will over rev and exhibit symptoms of an air leak.

By all means, take the precaution of warding off air leaks and RTV seal your engine anyways. This is always a good practice for all engines, new or used.

Another thing to check: if your engine still runs away after doing the above, dismount the engine, unbolt the heatsink and head, and verify that all the head shims that are supposed to be there are actually there. Consult your engine disassemply sheet to see that the factory installed the correct shims. If they are missing or there are not enough shims, you will have to contact the engine company to remedy this. They might even be good enough to replace the entire engine altogether!