Firstly you will need to know which version of Super Major because the name was used on at least 2 totally different design types of Merry Tiller.
Certainly the name was used on the mid 1970s type and then again on the last design type 1980s to the end of production.
The years go by and MT original spares are doubtless getting thin on the ground but I would still try locally at first... parts can sometimes lurk about still.
Locally, also being in Lincs, my nearest agent would be W. White of Willoughby, Alford. Other long established agents are RH Crawford of Frithville,Boston. Carrs of Boston. Lawnmower Service (LMS) of Wellingore, Lincoln. Fotherby lawnmower centre of Cordeaux Corner, Fotherby, Louth.
The drawings for one type the early 1980s to the end of production Super Major can be found as part of the Major diagrams on; http://merrytiller.dyndns.org/
Richard T' who posts on this forum may be able to help out...especially if this is an older Super Major...he operates www.merrytiller.co.uk
in support of of MT owners. and may have further info' on all types.
All the best,