Go-Kart Engine Compression: How To Check & Change?!?

I have a Yerf-Dog 3209 with a Howhit 7.8 HP, 150cc 4-stroke engine. The engine, about a year ago, started sputtering while being driven, and lost engine power. It has set for about a year almost, and today I pulled it out to get it going again. I replaced the gas filter, bought a brand new battery, and put fresh gas in. There was absolutely no gas in the kart's tank, so it's all fresh, even throughout the engine. When I tried starting it up, it would turn over, but it wouldn't completely start. Just the whine of the electric starter going over & over again, trying its hardest to start the engine. The spark plug is working (I tested for a spark), and there's plenty of gas getting through to it. The air filter is clean & in good order. The only thing left would be the engine compression, which I hadn't thought of before. Is there some way that I could check the compression on this small, gas powered engine, & how would I do this? (I understand to some extent it's a matter of using a gauge through the spark plug socket on the engine?). And if the compression is too low (It's supposed to be 155), how can I change & repair it? Is it just a matter of pulling the engine apart & getting to the cylinders? Also, I've heard of go-kart engine "jets". What are these, and would these play a factor in this? I've searched for go-kart engine jets, but all I get are actual jet engines (turbines). Thanks for your help in advance everyone!!