The likelihood of getting actual Stevens made replacements is low. You could ask Western Gun parts, Gun Parts Corp. Perhaps they have scrapped a 235 and have the parts. You could post in the EE.
You can buy matching sets of unfinished hammers which could probably be fitted. Check Dixie Gun Works.
Perhaps you could find a pair of hammers from a different gun that could be retrofitted.
If the gun is solid, and otherwise sound and serviceable, replacing the hammers could be an interesting project.
It would even be possible to make a pair if it came to that.