The logo is that of
Nippon Denso, and both from that and its age, you are basically not going to be able to find
any datasheet for it, much less one in English and not Japanese; my guess is that it's a mask-ROM MCU programmed as a frequency counter and calibrated to display speed. If you trace the schematic and figure out how the display is driven and where the input signal comes from, it shouldn't be too hard to replace it with an MCU of your own choosing (e.g. PIC, AVR, 8051, etc.)