It is possible to considerably reduce the oil mist blowout from the valve lifter by fitting an oil seal. This involves carefully turning out the fitting in the rocker box to suit the oil seal (a conventional sprung seal), and checking/adjusting the arm so that it does not rub against the seal. You need to strengthen the return spring, if using the original volute spring I used the alloy screw on cap of a sparking plug, with a hacksaw cut to fit it in place, and squeeze it closed when in position.
I can't remember the exact dimensions of the oil seal used, but a check in a seal catalogue (or ask a supplier) should provide something suitable.
A common problem with leaking valve lifters is wear caused by incorrect adjustment, ideally they should only touch the rocker ledge at the very end of their travel, only a small lift of the exhaust valve is needed to release compression pressure. If incorrectly adjusted, there is a danger that on valve closure the rocker arm can touch the lifter eccentric peg, and the loading will wear the spindle and allow/increase oil leaks. It will also make an annoying clattering noise!
I have also seen differences in the length of the eccentric peg, and if the washer/sealing gasket on the spindle carrier nut is too thin, the end of the peg can catch the rocker arm, if too thick the peg can slip past the rocker ledge, and in the worst case cause the rocker to jam and prevent the valve opening. If possible, check the clearances before replacing the rocker box, when the relative positions of the lifter and rocker arm are clearly visible.
I have seen the latter case when a rebuilt motor with a different exhaust rocker box was run for the first time, using the valve lifter to stop the motor prevented the motor from restarting.
Fortunately, new valve lifter assemblies are available from club single spares, or from John Budgen, if you find your lifter is worn.
Of course, if there is a lot of blowby from the piston rings, then this can overwhelm any attempts at making a motor oiltight.