For diesel the motor worm is the same thickness. 8mm. The key is the worm length. But I would keep the original motors because they are the better motor 385. Mounting tach should not be too hard.
As far as lights a PS-3 diesel board can not drive the bulbs as mentioned even with the correct chain file. They are not meant for bulbs only LEDs. Only PS-32 boards can drive bulbs because of the adapter board used.
There is no reason you can't ditch the CV board and go with the LEDs from the GP. Just match up the connectors. If you load a PS-3 F-3 engine sound file and chain file for an engine with MARs light you can get that to work too.
As far as bulbs, they are all 6V bulbs, but the HL is controlled by the old PS-1 board via circuitry on the CV board. That is the extra 2 wires brown and blue normally, that plug into the red connector. G
