guitars

Guitarfetish parts are perfect for a project like this. They are cheap enough that you aren't sinking a ton of money into an already cheap guitar, and are nice enough (given you don't buy their cheapest stuff) that you feel like you've made worthwhile improvements to that cheap guitar.

The biggest thing I'd say is to replace that wraparound bridge with one that has individual adjustable saddles. The way it sits now, intonation is always a compromise, and that's just not a good design from a performance perspective.

After that, the best upgrades you can make to a guitar like this are fretwork and a better nut. Assuming it still has the factory nut and fretwork, it's virtually a guarantee that they are shoddy, and you just won't be able to get it playing as well as it can without addressing those first. That's the big thing people tend to miss while upgrading cheap guitars and why they often end up unsatisfied with the results.

And finally after tackling those issues, then I would start looking into other parts like pickups and tuning machines. There are lots of options on GFS that are both cheap and decent, it's up to you to choose what you like. Check out some sound samples and the like. If you're trying to add a coil split, just make sure whatever pickup you get has four conductor wiring and get a push-pull pot if you don't want to drill any holes, though I much prefer mini toggle switches. If you're trying to add a second pickup, well that is very in-depth for a guitar like this. Doing it yourself will be a nightmare if you don't have experience, and paying someone to do it will cost more than the guitar is worth.

For Knobs you might be able to get some from Tayda: https://www.taydaelectronics.com/hardware/knobs.html

I order pedal knobs from there and they're pretty solid quality. You'll need to make sure you get knobs that fit the shaft diameter of the pot on the guitar. I'm not sure what it is for an LP Junior. The knobs are relatively cheap so you could order a few sets to try but the shipping costs will probably be $10 USD minimum and they don't do returns.

If you want to put in your own pickups you're going to need to learn to solder and de-solder. This is a good guitar to practice that on though. Watch some youtube vids on how to solder if you're not familiar and get a good soldering iron. If you can afford it I'd recommend buying a whole new set of innards you can assemble without potentially damaging the original parts. If you mess up you can always put the old parts back in.

There's so many pickup types out there. I don't think you can go wrong with Seymour Duncans.

As Cold Hotman said I wouldn't bother with the switch because you'll need to carve the body out for a second pickup and that can ruin the guitar. Coil tap might be possible but I've never looked into it myself.

Since its a single pickup, you dont really need a switch, unless you want to do some kind of coil split/coil tap kind of business. Personally, what I think would be cool is a tone control like this guy did that allows for single coil tones without an additional switch. Very cool.

GFS does good bang-for-your-buck pickups, and IIRC makes "plug and play" sets that require little to no soldering.

And are you talking about gold knobs like these ones (barrel), or these ones (top hat)

I'm not sure a new bridge would give you much.

For other hardware I highly recommend Wilkinson. Their tuners and pickups are excellent. I put a set of their EZ LOK tuners on my epi and really like them.

The 2 biggest improvements you can do are to get it set up by a professional and to gently sand the gloss neck down to a matt finish.

Gonna second those that have recommended guitar fetish. I did a very similar project on a single humbucker squire and I slapped a high output gfs rail pick up in there and couldn't be happier.