We inserted hundreds of these in the '80s and '90s without splitting the case. You need to remove the rods and tape up the oil gallery holes on the crank, set the case up on a mill, we used two old cyls. and shimmed it so it was true to the chuck and sealed the case up really well. You can make plugs for the open cyl. bores out of card board and tape, vacuum it off before you open it up again and flip it over. The cyl. sealing surfaces will need to be flattened when you are done so just remove .25mm and either add a additional shim or use a single .50mm shim. The engine is dry sump so if a stray piece of magnesium does go in the case undetected it will get caught in the oil filter. We have never had a single problem doing it this way. If send me your email address and I will come up with a few pics. of our set up and give you some more tips.
eisendudsinc@gmail.com