noob needs help with mileage - only 10 MPG!

Why are we talking engine swaps? This comes down to valve adjustment and carburetor before anything.

Renato, you can do a valve adjustment in less than an hour in the carport. I'm sure they just don't want major things like engine swaps and whatnot done in there. Just go buy yourself a valve cover gasket. Get the engine up to running temp. Pop the valve cover off and get yourself a ratchet with an extension. At that point just follow the valve adjustment procedure and it's wham bam and you're done. Put your new valve cover and gasket on and marvel at how much better your engine idles. Setting the valves to the looser side of spec makes them a little louder but better for mileage. Setting to the tighter side makes for a quieter idling engine but your mileage might suffer ever so slightly (talking 1mpg or less).

If you're still running bad, you're going to need a carburetor. You can get the best Holley 1920 rebuilder in the world and he wouldn't be able to fix a piece of junk remanufactured carb that's already too far gone. Swap on a NOS carb from a guy like Dan and you're guaranteed to not have any carb problems for years to come. Either way, even if you had to get a new engine, you're still going to need a new carb. Putting a piece of crap remanned carb on a new engine just means your new engine is going to act like the old one.