[Yosemite Sam]

Since we have no history on the engine that you just installed, then we have no idea what is/could be wrong with it.

That said...

If you used a feeler gauge to set your timing, go back and time your engine again using the static method found in the tech. section of this forum, however this is not your problem (and since you said that it ran fine yesterday, I seriously doubt that the governor is the problem either, but if you think you have too, then by all means adjust the governor). Once you set the timing it is set and you can't change the timing by adjusting something else. Governors are the same... Once they are set, they are set.

It is possible that there is an electrical problem, faulty coil, condenser, plug, plug wire or a bad wire or connection somewhere between the battery and the points.

If what you have told us is accurate (it ran fine yesterday, but does not today), and there are no electrical problems, then you absolutely have a fuel or fuel/air delivery problem. (Just because it is easy) remove the high speed needle from the carburetor, make sure the tiny holes in the needle are clean and free from obstruction and also make sure the tube between the tiny holes and the larger ones near the top of the needle is clear.

If this engine has been setting and the carburetor has had gas in it for a long time, then you need to go to your local auto parts store and get a 1 gallon can of carburetor cleaner with the "basket" inside, remove and disassemble your carburetor then soak it and all of its parts overnight. Then blow out ALL of the passages in it, reassemble it (using a new "kit") and try again.

Starting the engine and spraying some kind of aerosol product down the throat will not clean the carburetor.