Disregard compression for now. What about injector? Remove it then reconnect it and hold in a glass container. Wind the motor over and observe spray pattern. If the fuel "squirt"a fine mist all is OK. If a "stream" you have a faulty or clogged injector.
Fault codes can be difficult to follow but will only guide us to an electrical fault. Test the injector then we can dig further. PS higher octane fuel should not be necessary.
I have the downloaded hisun hs 750 crew service manual downloaded to my phone. I downloaded it for free, but I don't know where anymore. I would happily share with you, but I'm not very tech savvy and I don' know how to.