Exhaust has white smoke coming out and I dont know why...

[Power_uP]

I've never heard of either of those first two things, and I've never known your third option to cause white exhaust smoke.

Without knowing your level of mechanical experience, the best solution I can offer is to either take it to a local workshop and let them deal with it, or take it back to whoever overhauled the engine and tell them about the issue.

It could be a cracked head or a gasket failure. It could potentially be that someone skimmed too much off the head. Its unlikely that you'll locate the fault without removing the head though.