Re: bought a replacement engine and it wont start.
Congratulations on getting your engine running.
If your engine is revving up when not under load, then you have an air leak somewhere. Could be between the carb and the manifold, the manifold and the engine, or maybe the crankshaft seals.
To check the crankshaft seals,you have to remove the magnet and maybe magneto on one side, and on the other side, you have to remove the small gear. Look for any seepage of oil where the seal is in the engine block, and around those areas.
Did you adjust the idle speed screw?
There are varying opinions as to how to run the engine when it is new. Some say to take it easy, some say to kick its butt, some say to vary the speeds.
Make sure to run more oil in your mix when the engine is new for break-in purposes.