A simpler solution would be to use the same CPU done with a slightly newer process to allow a higher clock rate. That would ease the recertification process.

If you think they are at best 40 MHz 286 parts, the best that was available by then, running software derived from previous releases that ran on slower chips, there is plenty of room for improvement.

IIRC, you can get a modern 65816 (the 16-bit descendant of the 6502) in 300+ MHz. And I wouldn't expect it to be on a cutting edge process.

That considered, it's entirely possible most of the time is being consumed by reading the sensors. In that case, the sensor communication would need to be upgraded and recertified.