I sympathize with the OP - it sucks that the mower broke while his dad was using it. However, his dad has fallen on hard times - he's on disability, and I'm sure it took a lot to muster up the courage to ask his son for help - I know it would be for me.
If the dad is in a tough place right now financially, I don't think I could ask the dad to pay for the mower in full. I wouldn't even take the half. I'd suggest he takes the half and use that towards getting himself back on his feet.
It sounds like his dad tries to pull his weight around the house by doing the mowing. I'm not sure what kind of disability he has, but it would probably limit how much he could do physically.
Anyways, if the OP can suck it up, I'd do that for now. Sure, get your guilt trip in, but I wouldn't take the dad's money - mainly because he's the dad - raised him for 17 years, put him through school, fed him and put a roof over his head. That's more than the $400 it would cost for the mower. (not that one should be keeping track with family - I'm just trying to put it into perspective).