2005 Preditor 90 low Compression

I have a 2005 Polaris predator 90 that my 9 year old has ridden the tires off of. It began to smoke a lot last month and began running bad. It finally got to the point I had to cup my hand on and off over the air intake to get the quad back to the house. I did a compression test and it came back at 75 pounds. I purchased a rebuild kit on EBay from a cheap vendor with new cylinder, piston, rings and gaskets, after installing the components the bike would not start. I checked the compression and found it to be 45 PSI. After reading the forums I discovered that some members had challenges with the knock off parts. I took the new piston and rings fit it into the old cylinder and had 80 PSI. I have replaced the gaskets with an additional new set, checked the head for warp. Now the quad starts however it cuts out if you give it gas. After the motor is warm the compression drops to 68 PSI. What am I missing? Has anyone had this problem before?