I just ordered a 100cc kit from wish after shipping charges was 100 bucks
Be ready to tear it apart and go through it with a fine tooth comb.
At 100 dollars you are getting a factory second/refurbished or below par engine.
Is it a yd100 or ld100 or bt100?
I bet it's an 80cc (47mm) that's been bored out to 50mm, technically making it an 85/100cc.
FIRST REPLACE ANY CASE BOLT THAT ISNT HEX HEAD, the china girl stock bolts are notoriously known for stripping out.
Whichever one it is, check the exhaust and intake ports for obstructions. (Slag and casting debris left after being sleeved or honed from the factory.) Look in the ports where the carb and exhaust go in and out, see anything blocking the hole? Remove it CAREFULLY WITH a set of small files you can buy at harbor freight for 2 bucks.
Also, if you can, remove and dress or LIGHTLY sand or smooth the piston rings, they are pressed out and installed with no concern for smoothness or motor longevity.
And "dress" or file off the sharp edges on both the gears, the one on the motor and one on the back wheel to save wear and tear on your chain.
If it's the one piece cylinder and head, there isn't much you can do to adjust the "squish gap" ( the small gap left between the piston and top of the cylinder when the piston comes up and combustion takes place.) Adjusting the compression with a "higher compression racing head and porting and polishing are damn near impossible for a first time builder.
I know this is kind of confusing but by doing these few things, it will make the difference between a long life and a throw away motor. AND YOU WILL GAIN ALL KINDS OF POWER that normally you wouldn't see. ( Acceleration - low, mid and top end speed. )
I suggest watching every vid on YouTube about being a noob to motorised biking, it will give you the terminology you will need to ask specific questions later... And there will be lots of those trust me.
And don't be afraid to ask questions because these guys here are very knowledgeable and very very helpful and know their shhhtuff.