Iwave cube microwave is 600W magnetron output, however, it is no longer in production..
You can search for it if you like..
The only other possible way is to get a Panasonic INVERTER microwave.
They operate a bit different from standard microwaves.
Standard microwave ovens use a big heavy 60hz step up transformer to feed the magnetron. This setup has no way to control the output wattage of the magnetron.
Instead wht is done for "lower power settings" is the magnetron is turned on at full power then cycled off for a percentage of time. The result is averaging the heat level by changing the on vs off time.
Panasonic Inverter microwaves use an inverter to step up the voltages to run the magnetron. The oven can REDUCE the output power of the magnetron by reducing the voltage to the magnetron.
Somewhere (and I can't find the post now) I had run across some posts on a different forum discussing this and they had a chart with the power levels vs the input wattage consumed.
If I remember correctly, the 40% setting of a 950W Panasonic inverter microwave should get you down to a level that your 1000W gen might handle.
The downside of this is it WILL take forever to warm/heat/cook..
For instance, a 1200W output microwave will take about 1 minute to heat a cup of water to boiling..
950W micro would take 24% longer..
950W micro at 40% power is like a 380W micro taking 60% longer to warm/heat/cook
With 380W, you may as well use OTHER means to warm/heat/cook food that are much faster at less cost..
Those inverter micros do not come cheap.