Can you afford to hire an electrician to install a new one? Doing so would also allow a check that the wiring hookup is proper and in adequate condition so that you can be sure it operates safely.

Don't worry about matching the size. It's easy enough to buy a new one of any size and have a painter or handyman prepare the opening, patch the wall and repaint. Or, you can give it a try.

I agree with previous poster.

is there any info you can see showing the wattage and electrical info on the heater? This info will let you purchase a replacement that has the same capacity.

These are fairly strightforward products.