Hi cassenbl,
I hope all is well with you.
It's been a while since I've taken a look inside my generator, so I'm not sure what that could be.
I'm not sure if the newer generator will work with the BTSIS1. But if you are going to install a 900w heater, when you wire the element you can by past the heating controller on the cabinet, and wire the element directly to the plug. You will probably need to order a new short power cord with the proper plug ends, to connect the generator to the cabinet. Such as a IEC-320-C14 to IEC-320-C13 power cord.
Are you going to order a third party temperature controller to control the 900w element? I'm not sure if the controller built into the generator can control that many watts. I believe I recall someone posted how he wired his generator after installing a 900w heater.