Can I put a new fork on this?

I've just picked up an old tandem bike and I'd love to put on a new fork up front. Currently, it has a 700c.

Ideally, I'd love to put on a 29" MTB with shock, but I'm guessing I'm dreaming.

I don't know much about these old bikes, seems like it had a straight tube, so I'm limited?

From...