Do you have a picture of the part or can you find a picture of a machine that shows the part? Do you have the manual for the machine? Can you scan the page with the part on and note its part number?
From your description, it is very difficult to determine what you want.
You could try a wanted advert on homeworkshop.org (free) or lathes.co.uk (paid for) but a proper name for the part would be essential. If budget is not an issue, Colchester lathe spares (Colchester Lathe Spares) might have it, but they are mighty proud of their stock.
hi, not missing, front cut out to facilitate access to coolant pump instead of rear. Fare old thick lump of casting so I doubt if integrity or stability issues just would like it to be original!