KM141 or 140 gearbox parts

[Invicta]

I have a 1975 SWB Mitsubishi J26 Jeep made under license based on the CJ3/5B (M151).

I am looking to rebuild the KM141, 4 speed, gearbox as it is getting difficult to change down into 2nd.
As these Jeeps were only made for the Japanese Home Defense Force/home market I am assuming
that Mitsubishi didn't tool up specifically for this gearbox possibly using Spicer Dana internals?
(The engine is the original ASTRON 4G53 2.4 Ltr).

If anyone has any information on the gearbox/parts, even if another is compatible, I would appreciate hearing from you,

Bern