Of course trolls cant post photos, or make a single thing on a Monarch 10ee, somehow I can do both!
I am not limited to finding the chuck that fits. With the machine being a toolmaking lathe, carving out a custom back plate, including the pins, is a smooth operation, and better time spent then reading about "short bus drive follies" on this forum!
Anyway here is a 8" Union steel chuck,
Chucks just like this sell at HGR for $199 often, Monarch never offered a four jaw chuck with aggressive jaws like that, good chuck for castings, repairs!
Where the toolmaking part comes in, it is plainly visible in this photo that you can make your own back plate, but. not if you are a troll however. Do not expect chucks or back plates to fit your machine, that were not fit at the factory and supplied with your lathe. I found reworking import backplates as almost time consuming as making them from scratch out of steel. And they fit perfectly, that toolmaking thing again.
There are many good chucks that are rusty, look at the face and edges, pick a crispy looking one, clean off the rust, often 4 jaw chucks are seldom if ever used. Union Skinner Cushman BuckAronie!