Hi Vegoil,, I am so used to Listeroid engines that I forgot that the original Lister engines were slightly different, the cam follower on the end of the oil pump plunger shaft had a tee shaped hardened steel wearing surface pressed into it. Modern replacements do not have this. It should be possible to remove the original with a few sharp taps with a hammer on the square base plate. This should allow you to withdraw the shaft but will leave the small cam follower inside the engine, it may be possible to remove this with a magnetic probe by removing the crankcase door and working through one of the two holes in the camshaft drive gear, not something I have ever tried. If that fails you will need to remove the flywheel and governor housing.
Good luck,
Bob