Could be many things!
Air leak, carb ballance, low fuel pressure, weak mixture and totally of the wall is you gearbox free? Dose the prop spin while your running on muffs?
Is the engine stalling through lack of power or is it mechanical.
Another possibility is head gasket, but don’t get carried away!
start by checking all the simple stuff first,
Air leaks around the carbs and inlet manifold can be checked by spraying WD40 around them while the engine is running, the engine will pick up momentarily, check the diaphragm in the fuel pump. Reset the mixture screws,
Its all fun stuff and you get to do it six times!
the best advice i can give you as someone who wants to learn is get it serviced by someone who knows what they are doing and then once its up together learn to keep it that way!
otherwise with that motor there is a chance you'll go mad