I recently mounted a pump that supposedly was rebuilt a couple of years ago
on a 1.6D in a fairly unknown condition, it starts well, runs well on idle but when I rev it up enough it starts to misfire and produce lots of white smoke...
Could it be something has jammed in the pump? Or maybe bad injectors? Also there seems to be too high pressure in the coooling system when the engine has gotten warm enough so I was thinking that it could be a cracked head or leaking head gasket too.. Oh and the cold start was stuck in its activated state, was able to push it back though after opening the plate on the front of the pump.
The pressure in the coolant is usually a blown head gasket and perhaps a warped head. If you get pressure in the coolant on a cold engine that's only been run for 30 seconds or so, the gasket is definitely shot.
The rough running could be the injectors and pump are gunked up with age and abuse, just run some ATF or concentrated diesel fuel additive through it to see if that helps.
no pressure when its cold, only after the thermostat has opened.
Well that's good. If there is not enough pressure in the cooling system to pop the emergency vent in the overflow tank cap, then the pressure is not too much.
Sounds like it will be fine if you figure out your fuel problems.
it pops when it gets warm
it pops when it gets warm 
Is the radiator fan coming on when it gets hot? Maybe it's just plain old overheating. Could be a weak cap, too. Old caps sometimes pop too early. A new cap is $5-$10, an easy test.
Pulled it apart today, leakage in the head gasket, water in the oil and also the block is cracked so I need to find a good block..