Welcome!
I have quite a bit that I can share with you about this bike, since I worked on Son of Weldangrind's Dirt Runner 49 a lot. Your bike frame is cloned from the Yamaha PW80, and the engine is a clone of a Honda horizontal motor. The frame of architecture of the motor is the 50 / 70cc motor, and your model has been bored and stroked to nearly 100cc. FYI, you can bolt in a 125 or 140 if you choose.
Look for the actual displacement to be stamped on the cylinder. The cylinder head is silver, and is where you'll find the spark plug. The cylinder is right below that, and it's black and finned. Don't be surprised to find a number like 88 on it, which means 88cc. Most motorcycle manufacturers have been guilty of rounding up. A 110cc engine is actually 107cc.
The leak looks to be coming from the cover for the automatic clutch adjuster. Remove the two screws and the cover, and I presume you'll see an o-ring that's worn. Just buy a matching o-ring from a hardware store, or an assorted o-ring kit from Harbor Freight.
If you need to order parts, the easiest source is eBay. You'll find ridiculous amounts of stuff for your bike. Don't bother with brand names; just search for things like "125 dirt bike carburetor", and you'll find what you need.
Yes, please tighten those spokes immediately. When the spokes are at an adequate tension, they'll ring when struck with a wrench. The act of tightening the spokes to correct a wobble is called truing a wheel, and it takes some practice. Search Youtube for vids on it.
Yes, you can install a key switch. Find a cheap one on eBay that you'll be able to mount somewhere, and let us know once you have that part sorted out. We can help you connect it from there.