Qualcast Suffolk punch aluminium engine head bolt torque

I have a 43s with a 98cc aluminium engine. It says electronic ignition on the fan housing and it has a delorto carb.

I have removed the head and lapped the valves in. Following reassembly, the head gasket was blowing.

i have ordered a new one, but does anyone know the torque setting for the head bolts? Since they are going into aluminium, i don't want to strip the threads in the block.

john