I ordered a ebay oil tank for my DT2 but of course it's not for a DT2, as it has a banjo fitting and the sight glass is rear ward facing. It does have a brand new sight glass installed and I need it for my CT3. Any way to get that out to reuse?

Try pressurizing the tank with air? Don't know if it would work or not, but it's all I can think of that might work.
Tape plenty of rags around the glass to safely catch it if it does come out.... and always keep in mind that pressurized steel vessels can explode if over-pressured. 
Fwiw, these folks will sell you a new sight glass at a reasonable price. 

sumorubber.com/
 

The DT1 has a banjo and seems to fit your description. Why not send it back and buy a new sight glass?  

Hi. I've just joined as I'm restoring a '76 DT175 for a friend. We didn't know this site existed, just stumbled across it on Google. I removed the sight glass by warming slightly with my old hair dryer (workshop use) then pushed from outside with a long socket that fits just inside the rubber. Fell in the tank but retrieved pretty easily.
The problem now is: how to push in a new one? I'm hoping someone on here will be able to help with this?

Installing my new one was very easy after putting a light film of oil on the outer surface. Didn't take much force at all. Popped right in there.

So you need to remove the new one & fit in another tank?. Can you heat it to soften the plastic/rubber stuff & flick it out--carefully of course. Then reheat if neccessary to fit it in. Not sure if it needs sealer but i'd think they're designed to be tight without. But then again, in & out & back in--sealer.

Yep, heated it up with heat gun and popped right out and into new freshly painted oil tank