Mark
I can't help on your repair problem, but there appears to be an obvious short term solution. Don't use the heater!
Depending upon your pump type, you would typically expect it to have a safe working tolerance upto about 60C (this is hot enough to scald the skin) so fill with hotish water.
Another option is to look at your working practice. Will there be just 2 operators with no casual? In cases like this, I find it more productive to have just one machine working with the assitant pre-vaccing, spraying agitating and ferrying water. production rate is normally greater than double of one person working alone.
Next option would be to using cleaning solutions that are not dependant on elevated temperatures, although if you are working on, say, greasy restaurants, it may be slightly more time consuming.
My final option for consideration would be to use one extractor, a rotary with pads and a microsplitter. Spray everywhere, agitate everywhere with pad (Charly's would be ideal, but a combo of brush and cotton viable). You will have cleaned light/non trafficed areas with the pad and just require a rinse on the brushed traffic/soiled areas.
Safe and happy cleaning
Ken