how to fix my aprila sr50?

i have a aprila sr50 ditech 02 reg,when i run the engine for about 10 mintues it keeps over heating its a liqiud cooled engine,i have checked the thermostat thats fine,checked head gasket thats fine,but when i but the head back on its leaking water.so itook the head backoff made sure i put it on propley but its still leaking.please help.