If only using in the summer perhaps you could temporarily by pass the heater altogether then repair next winter?
I did that for two years with my other one using the barrell of a cheap copy AA Maglight which I've kept in the boot ever since in case of a repeat.
Weather permitting, I'll let it run under strict supervision today but I have to wire the cooling fan directly as it doesn't seem to come on itself.
On an entirely different note, i fitted a new in-line fuel filter when I changed the rubber fuel lines a few days ago and it was very difficult to get it to start afterwards.
It just didn't seem to be able to pull the fuel and I had to use a hand vac. pump to get the fuel through.
When I reversed the hoses on the pump to the CORRECT orientation, it starte surprisingly easily. Doh!