That washer on the plug wont seal, your going to have an air leak. Is that red sealant silicone? If so, it wont hold up to temps and gas. Use Hondabond or better a gasket and figure-eight the head and barrel top on a granite stone or cooktop glass with 320 wet sand or emery sheet. For a custom build a leak down test is a good idea.
Also some mower and chainsaw plugs have shorter threads, you can match something close in a hardware store just to try out as the spark should be in the right place in the chamber. Too close to the piston and it will see less atomized fuel and foul out. (edited)