like this the change wheels will be missinghttp://www.lathes.co.uk/drummondphotoessay/
What model is it? A photo would help.
The early ones had a power shaft driven by gears at the headstock end which went to the other end and drove the lead screw through a pair of gears at the tail stock end.
On mine the power shaft was missing
If you're going to be missing some gears, those would be the type to be missing as they are a common size to quite a few lathes.
Personally I wouldn't put too much effort into restoring a B type, as a novice I'd look for a more complete and more modern lathe.