Priming the engine before start up, normally it only takes like 2 - 3 pulls with the exhuast covered. This time it taks a lot before the fuel reaches the carb. When I prime it by blowing air into the pressure line, the fuel reaches the carb but then backs out.

What could be the problem?

Can I fit a carb from a different engine? If so, which one?

If I'm not mistaken, you can use the carb from the XTM .28. About your problem with your 24.7 carb though, there could be build up in the carb. Are you running a fuel filter, because I had this problem when I wasn't running a fuel filter. I took my truck to the LHS, they blasted the carb out with some nitro cleaner, and I bought a fuel filter. I now run duel fuel filters and haven't had the problem since.

It has a stone filter inside the fuel tank, which I flushed with nitro cleaner going the opposite direction to clear up any clogging.