The 'trick' to oversized pistons with brand new seals is to put the piston shaft in a power drill and sand it down to size with glasspaper or similar. This means reducing the diameter of the seal very gradually, since if you do too much it's irreversible!
The reason why you soak the seal in silicon oil (some people do it it water) is so it swells as much as it's going to before you start resizing it. The correct seal size is so that the piston will just fall through the cylinder under gravity, and will stop when you cover the transfer port with a finger.
If you soak the seal in engine oil, you risk the rifle 'dieseling' for some time after you get it up and running, as the oil ignites under compression. Use silicon oil for leather, but be careful you never use silicon oil on its own anywhere for metal on metal contact - which is a disaster.
PS The trigger group on my (former) mod 70 here:
Looks very like the trigger on the mods 80 and 90 (though I never dismantled it).