I've taken apart a few folders, and all have had their lenses shimmed into place. You would most likely have to do that if you did a lens swap. It's not hard in principle: just put scotch tape over the film aperture, point the replacement lens (same focal length as the original) at something really far away and open the shutter (T or B setting), get a dark cloth and stick your head under it so you can see the image on the tape, and shim the lens until it looks sharpest. It's tedious going, and maybe you'll be very unlucky and back focus for your new lens would mean taking off material, rather than adding it.
Or just use the Hapo as is. I have one, and that Enna is a real sleeper, it's very very sharp even wide open and takes great pictures. The only things I don't like about the Hapo 66 are its fiddly film advance and the fact that they never made a Hapo 69 (I would be all over that in a heartbeat!).
--nosmok