What would it take to rebuild an appliance for 120v
This could be done without modifying the Tubie.  There are two primary considerations:

Power consumption:

The power consumption is greater than what is typically supported by residential wiring in the USA.  Our receptacles are generally rated to 15A @ 120V, which is 1800 Watt.  There are homes with 20A outlets and wiring, which can provide 2400W, but this is less common.

As you said in your comments, the two options are 1920W or 3320W. So even if you bought a transformer you couldn't source enough power through a standard outlet. And transformers that can handle this amount of power are hundreds of dollars.

This can be overcome if you use a 240V outlet.  If you have an electric laundry dryer, this is 240V.  Perhaps you have an additional 240V receptacle already wired in your garage or basement.  In this case you wouldn't need any sort of transformer; you could plug right in to the outlet.  240V is close enough to 230V rating as to not cause problems.

Mains frequency:

A second issue is that we have 60 Hz AC instead of the intended 50 Hz.  This won't effect the heater or power consumption.  But it will likely affect the fan!  Perhaps the only difference is that it will run slightly faster.  But it may also be more prone to overheating or stalling.

If it turns out that the fan has problems, it is quite inexpensive to buy a small 240V-120V transformer that would only support the fan.

Fianlly, it is possible that the timing circuitry is based on the AC frequency, but this is getting to be pretty uncommon.