Converting 3x7 to a 1x7. Is it possible with only existing parts? I own a 2000 model Specialized Ground Control full suspension. The front derailleur is rusty and temperamental so my solution is to remove it as well as 2 chain rings (they are removable on mine) and convert to a 1x7. This bike is not worth high spendings on upgrades so would this work if I just bought a 1x drivetrain specific chain ring and kept everything else stock? Better yet would it even work on the current middle size chain ring with maybe a chain guide attached to keep it held on? If spacers are required that's all good. I am only a novice MTB rider and the bike will predominantly be used for downhill and small jumps, no hectic double blacks or anything. I am aware that 1x7 isn't very versatile but it will be plenty for me to have fun.
You have a zero cost solution, which is to keep the front derailleur but remove the cable (and shifter if you like). Then use the FD limit screws to fix it in the right position to act as a chain guide.

You don't need a 1x chainring, in fact using one with a 7 speed chain is probably less than ideal, so you're actually better off sticking with an existing chainring. You may not be able to get rid of the big chainring, or you may need to get shorter chainring bolts if you do, depending on how the big and middle rings are mounted.