GV150 HR21 honda 1978 mower shuts down - HELP!!

I have a GV150 HR21 Honda 1978 that was my deceased father in laws that my son and I would love to resurrect. It runs great.....for five minutes! After five minutes or so, it just stops. I was lead to believe it might be the CDI ignition coil. They are hard to find, but we finally did find one. Although also from a GV150 HR21 mower, on the spark plug cord it said "-1" as opposed to the original one that said "+2." (Could someone explain this to me?). Regardless, when I put the new used one on, no help, same problem. Runs great for five minutes, then it stops running. If I go out 10 minutes later, ya baby, it starts and runs....you guessed it, for five minutes. So something must be overheating, then shutting down, then after cooling, it can run again.



So HELP! We wanna make grandpa in heaven proud. What is our problem?



Thanks. Gregg