Well the first thing to do is clean off that rust and get the parts oiled.
You can sell now, or later. WW2 parts are dried up and getting expensive. Top covers, bipods, grips, and flash hiders have doubled in two years.
You can build it yourself, with patience. And if you're willing to learn, and wait for the right parts, and have a friend help for a few bucks it will be a lot less expensive.
As you know, you're missing the front nose and whole rear of the receiver. So to build there are two roads to take: (1) original german or (2) just build to shoot. #1 will be worth a lot more money for the same amount of work, but it will take longer and be more expensive.
The chance of finding just the front nose for WW2 is close to zero, but not zero. If you wait long enough you'll find a complete front end, albeit with the nose (bearing) torched. The WW2 rear parts do come up; they run $600 - $1000 (or more!) right now. If you're serious about it you'll want to match the rear (mfg & date) with the front. The big thing here is patience- you can buy what you need on GB but it's going to hurt, or you can watch GB and talk to people and find what you need at a far less expense.
If you don't care that it's all WW2, use a whole MG3 nose (sight ears, etc.) now going for about $250. Then you either find a M53 rear (low probability, $600) or aftermarket shells ($300 plus maybe $150 in other parts) and have a machinist/ welder help you stick to the blueprints.
After the receiver is "done" you need a semi grip and bolt. You can get these from BRP (~$750 IIRC) or modify the one you have with a mill and add AR15 FCG.
No matter what, when you have the receiver all welded, I'll tell you the same thing I tell every excited owner: "Congratulations! You're 1/3 done!". These things need work, tweaking, testing, fitting, filing, etc. It's dozens and dozens of hours...and frustration. Building an MG42-type is not like an AR15, FAL, or even 1919a4.