objective lense? Answered

In an unfortunate coincidence, after reading your question last night, I went to my local rubbish dump today only to find 15 overhead projectors and slide projectors all dumped in a heap. I tried to prise the lens holders out of a couple of them, but had no tools or time. Scavenging is not allowed at this place ,so I asked the attendant on the way out if i could grab a few (all) of them and was told no (they have security cameras), and they were about to be crushed into the ground by the compactor.
Some of the stuff people chuck out would almost make you cry. Sorry for being of no help whatsoever.

You didn't list the model number so I can't tell whether parts are available or not.

Kalart Victor is out of business.  But they sold the overhead pro. portion to NEUMADE in the 80's and they are still manufacturing the same designs and can supply parts for them.  Google "neumade overhead parts" and you might find what you want.

YOu can google KV and add parts to the search and there are people out there still supplying parts for some of them.

You might find a broken projector that still has a good lens on Ebay cheap.

Even though you know the size of the lens you still need to know the focal lenght to be able to buy one from a lens supplier.

It might be a good idea to just look for a working overhead projector on Ebay instead of trying to repair the one you have.