First some terminology...
1. If the pit and pump go outside the house and not into the septic system it is a sump pump.
2. If the waste runs to a pit and then is pumped up to the outlet into the septic tank it is more often called an ejector pit.

Take a look at your pit. Is it a plastic tank witha pump in it? Does it have a cover? This pit may work fine but just be missing the cover. Covers can be purchased at most home hardware stores. Just make sure it fits tight and seals the pit. Also you want to have your ejector pit vented. This is really nothing more than tying it into your current waste system venting.

I just finished such a project and the one thing you didn't mention is venting the bathroom plumbing. This can be done without tying into the existing plumbing venting. I used a check valve type of device. I can't remeber what it is called but as water drains it pulls open the vent and then when the flow stops it closes so air can get in to drain but when no water is going donw the smell doesn't get out.