Just to add a few more tips.
Don't be a silicone junkie. If you clean off the old gasket as recommended you shouldn't need a sealer.
An old product called "Gasket Cinch" will keep the gasket attached to the cover while you are trying to get it on. Gasket cinch is aplied like contact cement, coat the cover(always attach the gasket to the removable part) and the side of the gasket that attaches to it and allow to dry and then using a couple of screws to aling the gasket just press. Gasket cinch is a low stick kinna stuff and will allow the gasket to be removed later with little effort.
Now you have the plate with the gasket attached just reach for your tub of BTG (Boat Trailer Grease). Go ahead, get a little on your finger and rub on the side of the gasket that goes to the housing.
BEWARE of rubbing grease on the housing as it might have a sharp edge.
As most mechanics know--If you put the first bolt in at the 12 o'clock position the cover will almost swing into place. Remember to always start all bolts before tightening any and then in a criscross pattern.
The next time you have to change the impeller the gasket will come off with the cover and be easy to remove and clean up will be a snap.
If that day comes when you have to change the thing in a hurry just pull the cover change the impeller and use the old gasket and you should be done in about 5 min.
If of course you have one that is easy to get to