Electronics in high temperature environments
This is well into the territory of "hire a specialist design firm". It's going to be expensive, so you might as well do it properly and hire people who know where all the pitfalls are and where to get all the parts.

TI have some high temperature microcontrollers, and a design guide linked from that page. The SM320F28335GBS is good up to 210 degrees (which is cutting it a bit close). While you can buy from Digikey in single quantities, they cost $300.

The other approach you could take if the operating time is limited to a few hours in that environment and the box can be sealed with no wires in or out, is to insulate it very heavily. Add some thermal mass, preferably with a phase change. Do not exceed the stated cooking time.

You'll end up with a something like a meter cube of fiberglass or foam insulation containing a sealed shoebox of methanol with the electronics floating in it, which will stay below 70C until it boils dry, hopefully for long enough for whatever your heating cycle is. At which point you need to cool it down again equally slowly. I think that's the only viable strategy for normal temperature electronics which doesn't involve active cooling.

Edit: normal batteries do not like high temperatures either, although there are special high-temperature molten-salt ones used in some missiles.