low compression p38 2.5td

Hi i am just looking for an answer i had trouble with starting my range rover it was an ongoing problem first it started having trouble starting only when hot,so i fitted a new FIP pump,this cured the hot starting problem,so all was good for a while,then coming along motorway one night it dropped a cylinder,removed the head and found damaged valve replaced valve and then redone the cylinder head,everything was good back up and running and running great,then 3 months later started having problems with starting again it took ages to get it to start,when i did get it started it would run fine,but there was signs of back pressure out of the dip stick and when you removed the rocker cover oil cap,so i carried out a compression test and all cylinders are down on compression so that was the reason for the back pressure and the trouble with starting,my question is can you do the piston rings without removing the engine i have the head removed,i was just wondering if it is possible to remove pistons with engine in place or does the engine have to be fully removed.