hi Steve! congrats on getting the blockage sorted.
Where abouts are you? Im in Uxbrudge NW you said you are collecting exhaust from ricambio which is in south London (surrey).
With regard to your gearbox prob, it doenst sound like clutch at all,
on the drivers side there are two bolts on the centre consol tunnel these can be slacked to allow the re aliging of a small plate whic allows your to select the gear, If the metal plate has moved it can prevent it from selecting the right gear, and as you say you cant get reverse it goes into 4th instead im thinking this could well be it. If it is gearbox a new gearbox is between £50/100 again labour will be expensive.
The exhaust is very easy to change really and if you dont do it im sure a small garage will. Get some WD40 onto those manifold bolts a few days before you try and remove them.
Seeing as your tank is so rusty it will be worth fitting a fuel filter before the fuel pump this will stop any blockages in your pump or carb.
hope this helps mate, lovely looking 126
regards
Chris