You can easily install modern locks into an old door, but that would be an awful thing to do, in my opinion. It would really not look appropriate in an old house.
Check eBay, craigslist and local flea markets - you can easily find old mortise lock parts(often in big lots) Sometimes it might even be new old stock. Those parts would be of much higher quality than the crap they sell nowadays.
I'm actually in process of doing pretty much the same thing in my old house. At some point one of the previous owners installed hollow core 6 panel doors throughout(not only they don't even deserve to be called doors IMO, but also they absolutely don't match the house style) I sourced some reclaimed solid 1 3/4 doors and have been installing them instead. So far I've had good luck finding appropriate vintage mortise lock sets on eBay.