You can do away with the speed sensor & plug the hole, the relay kit I bought from Devon4x4 doesn't use the sensor. But you have to remember to disengage OD below 30mph.
Quite likely as GKN don't make that OD anymore & Devon 4x4 bought all the spares stock & doubt they will have parts made when stock is exhausted, Vulcan Bomber on here has diagrams for all the gaskets.
The gear knob lower part should be esdy enough to get made is a alloy machined part, the lid with switch & led should be available from Devon 4x4. Or just find a on/off switch to fit the dash or on gearlever.
There is a gkn strip down thread by reb78 in the Defender forum that will help you strip yours apart.
They use ATF only, don't use anything thicker.
Also in out tech archive index is a link to the owners handbook & fitting instructions,
I bought the MO version but it doesn't fit if the transfer box has the bigger sump on, the OD & TB clash, they both need part of the same space, so it either one or the other.
As the 300tdi engine moved up & further forward it needed a longer cable, so the original short cable was discontinued, as the long one fits both engines.