My guess is that the idler pully has seized which has caused the belt to break. Becasue there are no valve springs (only a light helper spring) in theory it should be possible to turn the engine over by hand (with the plugs out). When the belt broke i think its likley therefore that it didnt stop with all four valves closed becasue there isnt much spring pressure on the cam to make to turn that little bit more. So i think that the piston has clouted the valves good style which has bent something in the head which is preventing you from turning the cams. You need to run the valves along a piece of plate glass to see if they are less than obvioulsy bent.
Has the bike been standing for a while? Has it has a recent belt change? Do the belts have white writing on them (old glass fibre reinforced ones) or red writing (newer kevlar fibres)?
Did it come from a dealer or a private seller? I'm guessing private because you are having a go at fixing it your self.