Could you please tell me the length of the cutting dect belt on the Boland ST140 mower,

Hi,
The Bolens ST140 was in the Suburban tractor classification. It could conceivably had fout different decks. The only place I go for reliable info is usually Sonny's. That said there are a few other sites that have some info that is often gleaned from there. It is at:
Bolens ST Series
That is the deck and machine page that links to manuals for machines and decks.
Peace,
Clarke