I used a brass brush by hand to clean the carbon off the aluminium head of my BMW R80GS 1991. The valves however are very stubborn, a brass dremel head won't do it either. I started using a soft steel dremel head which works a bit, but I'm afraid of damaging the valves. I believe the valves are hardened steel and not so easily damaged? Also I would be careful and not get to close to the valve seats. There I use a brass brush by hand. I've also taken some of the chunks off the valves with a small screw driver (very carefully). Is that OK?
I've considered ovencleaner but cannot find anything not containing sodium hydroxide (aggressive for aluminium).