Slash, bit time consuming but remove the carb and exhaust and plug the holes with a piece of rag in each port. Make sure the piston is covering both ports. Small paint brush and a scraper/knife and sit down and work your way around the barrel, scraping and scratching, time consuming but about the only way. When you think you have it all clean, give it a final brush down with plenty of kero on the brush and then hose it down with your finger on the hose to get a decent jet. Make sure the rags remain in place. After hosing down remove the rags and give a squirt of WD40 in both ports and sit the motor in the sun to dry. Once dry you can mask around the head and the cases then spray the barrel
