1- wait till its cold and check for any leaks.
2- take the bottom radiator hose off from the radiator too dump all the coolant. Then use a hose to flush out the radiator and engine. Refit the bottom hose and tighten the clamp. Next remove and clean the coolant overflow bottle thats near the battery.
Fill radiator and overflow bottle to max mark with coolant(tectaloy 90 or similar 5L will do) undo the coolant bleed screw located on the inlet manifold as ur topping up coolant, once coolant bubbles up from hole refit bleed screw and nip up.
Then start the engine with the radiator cap off(NOTE: only take radiator cap off when the engine is cold) turn the heater on so that coolant flows through and let the engine warm up. The cap should stay off for a few minutes to let any air escape. Refit cap and go for a sensible drive to get to operating temp. Go back and turn off and let it cool down. Leave to cool andthen recheck the coolant level in the radiator and the overflow bottle.
NEVER OPEN THE RADIATOR CAP WHEN THE ENGINE IS HOT.
3- try for a rwc and see if anything fails.
4- i use threebond engine sealant. Its great stuff, make sure mating surfaces are clean and flat before using the sealant. Apply a thinish layer of sealant and mate surfaces together and bolt the part on. Check for leaks after running
Hope this helps