strutchr, you are in the right place, as trusty220 has said I am the man with the Monro information. I have sent you a pm. I have owners manual including parts list for machine and engine. Look after those tyres as they are an obsolete size and no longer available.
The Monro Tiller series 3 has 3 forward and reverse gears, the earlier series 1 and 2 machines only had two forward gears and no reverse. Ratchet hubs are fitted enabling one or both wheels to rotate freely. 4 different types of time were available for the rotavator and there were a selection on other attachments including cutter bar, cylinder mower, pump, generator, trailer etc.
The original engineering drawings are in the archives at Museum of Rural Life (MERL), Reading, where I have been helping to catalogue them.
Not to sure the Monro is any lighter than a Lister D but it does have the advantage of been on wheels.

If the video is the one I think it is, that is my Monrotiller and trailer (you tube name rototiller56).
If you are having problems signing up for paid membership this is an alternative link https://vhgmc.co.uk/mem-reg/