I own a Scamp 4x4 based on an F50 dating from 1983/4 ("A" reg) fitted with a 5-speed gearbox.

The gearbox has recently been replaced (previous one didn't like second) but with this one it sometimes stays in one gear (first or second) when changing to reverse, thereby selecting two gears. The only cure is to remove the gearstick and disengage all gears and start again. I'm pretty slick at this procedure and have got it down to ten minutes!
The gearstick came with the box and my old one looks the same and doesn't cure the problem. I didn't notice this problem before it was fitted (obviously!) but have no way of knowing how or when it occurred. Seller claimed it was alright previously (again, obviously) so something may have gone wrong since.

Is this a familiar problem for which a cure is known?

If there is no easy fix for the gearbox, I may take it as an opportunity to update the vehicle by fitting a later 2.8 TD and replacing the entire loom / dash at the same time. This may sound extreme but the wiring needs to be sorted as some elements have become unreliable.
Will the later F70 engine and box fit into the F50 chassis using the F50 drive shafts?
If I go this route I will probably but a complete, running, rusty donor.

All opinions and advice would be much appreciated.
Pete