Get yourself a tub of petroleum jelly and put heaps on the lifter, that will hold them in place whilst fitting the cam.
The petroleum jelly will dissipate when you start the engine and get a little heat into it and not cause problems with blocking galleries or anything like that.
This is what had to be done to refit Renault 10 & 12 heads.

Have you ever heard the saying. Pushing #### Up Hill. Cos thats what you are doing by not to pulingl the motor out. MLS has the best way to hold the cam followers in. But if you have to pull the sump off you have to undo the engine mounts so you may as well undo the dozen or so other things & pull the motor out. If you havent got the equptment to do that you can hire engine cranes or get some mates to help lift it out with a pole & chain & if you havent got a engine stand sit the motor on a couple of tyres & get a mate to hold it while you put the cam in. Its not hard man.