Help on replacing an inbuilt microwave oven?

I have an old Sharp Carousel (model R-9X55) that was in the house when I moved in 13 years ago...I think it might date back to the late 90's! Kudos to Sharp on building something that's lasted so long...but sure enough she's now dead. The service centre tells me spares are limited given the age, so I guess it's time to update.

The problem is, it's built into the kitchen cabinet, above the oven. I've done a ton of web surfing and can't find anything that seems even remotely close to fitting into the cavity, so I'm guessing these things aren't exactly standard? The case dimensions of my current one are W/H/D 627/381/527mm.

The cavity measures 640x400mm, but there's metal rails built into the bottom of that, which the microwave's feet match to...i'm guessing that's going to make it even harder to source a replacement.

Would you normally need a builder/kitchen cabinet supplier to come out and change the cavity to match a new microwave?