Mitsubishi 3000GTJ26 Jeep - Mitsubishi KM141 Gearbox problem please help - InvictaJ26

The 2nd gear on my 1975 Mitsubishi Jeep is getting difficult to engage.
Incidentally the KM 141 is column change version of the KM140. All parts are Mitsubishi original.

It is OK when the vehicle isn't running and all other gears engage and disengage alright as does the Hi/Lo box and Warn Overdrive.

Please can you suggest a suitable gearbox specialist who could remove, diagnose and fix the fault and then refit preferably in the West Midlands/South West area as I live in Tewkesbury. Due to the rareity and age of the J26 Mitsubishi dealers are not my first option.

All help will be very much appreciated as otherwise it is great fun to drive especially now the weather is warmer and I can have the screen down and the top off.

Many thanks,

Bern