The original microwave was either an Origo or Tappan, about 0.5 cu ft and 550/600 watts or so (different name brands, same microwave). Unfortunately, microwave was discontinued years ago and no replacement microwave with the proper dimensions to fit through the cabinet opening and air space is probably available.
The cabinet opening of about 18" x 10" (curved corners) restricts the possibilities.
Option 1. Replacement microwave.
Unlikely to find but would be nice. Consider the dimensions without feet or handles (every little bit helps, replace feet afterwards).
There may be Westy owners who do not use and may be willing to sell. Can also check ebay, etc., for one.
Option 2. Repair.
If lucky, might find a repair shop. Worthwhile if can't find replacement.
Option 3. Enlarge Opening
If can find a microwave which fits the interior space, can enlarge the cabinet opening (get rid of folddown cabinet door).
Possible searches for "micro" or "mini" or "compact" or "small" or "smallest" microwave brands might include Haier, Daewoo (a QT1 240V version used in Europe), and portable Wavebox (not good reviews). Have to confirm specs.
Haier, Danby, Proctor-Silex, Westinghouse, and other brands have small 0.6 cu ft microwaves which are slightly too big for cabinet opening (but may fit if feet or cabinet opening is enlarged).
https://www.amazon.com/dp/B00BGTO1WC?tag=smallest-microwave-20
240v version:
http://www.daewooelectronics.co.uk/14l-compact-manual-microwave
Wavebox might fit without carrying handle?
https://www.amazon.com/WaveBox-Original-Portable-Microwave-Cobalt/product-reviews/B002P6NG0S
Option 4. Use Space for Something Else
Use the space for storage, like a toaster oven (have to remove when in use and cooldown before storing).
If must have a microwave, there is a small Whirlpool rounded cube microwave. Too tall to store in that space but could store elsewhere when not in use (behind benchseat).
https://www.amazon.com/whirlpool-sm...8&page=1&rh=i:aps,k:whirlpool small microwave
There are several small concept microwaves which apparently never made it to market, like
http://www.wayvtech.com/
http://www.beanzawave.co.uk/
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Please post whatever you find and do. Thanks.