strange noise / engine change

hi been out in my p38 diesel today and it started to make a funny noise so limped it home, had a quick look over it and think we have come to the conclusion it could be a tappit going/gone, on tick over it sounds like a top end rattle when the revs rise it disappears? so going to take the rocker cover off tomoz and have a look, any other things i should look at while doing that and has any one any idea how long it would take to take the engine out and put another one in as that might be a option thanks