I have used a couple of there ring sets, also there inlet rubbers. Good gear and the Gurls blouse guys like them. There oversize piston and ring sets are good prices if your bores are worn.
I would agree with the vacuum pipe from carb 3 to fuel tap also. Block that pipe and turn tap to prime to see if it runs on 4 cylinders. Don't leave tap on prime without engine running. Check the tap works as it should and shuts off with engine off.
I had a similar issue, when I removed the switch the brass disc was broken. Fitted a different one and all good. I do have a low pressure guage that connects to the oil gallery on right bottom of cylinders.