There are generic, easy to find walking feet for the old Husqvarnas. You can some times find old Husqvarna branded attachments too, but they usually require a bit of search and patience to find. I think they recommend you not to disassemble the bobbin case completely on your own, if I remember correctly it requires special tools to get it back in place correctly; so clean and lubricate around the parts best you can. I think the top of the free arm comes off with a few screws. I can't remember about twin needles, but I think both model 19 and 21 takes them. I know the later models with much the same rotary bobbin case do.
The only tedious thing I remember with this model can be the reverse button. It's a combo push-turn botton, and if it's frozen it needs repetitive applications of oil in the back, oil, push, turn, oil, push, turn, test sew, ... In the worst of case it can take a few days with special attention and a good oil to get the parts shifting smoothly. It's not difficult and long as you are aware of it, only a bit time consuming. If it's stuck in reverse, lubrication and freeing up parts are the solution.
The motor is strong and fast, especially compared to many modern machines. The motor belt is easy to sort out, if you are lucky, the machine has the two gear pulley. DIY service isn't as easy as with the old straight stitchers, but there are expert help to be found on the web and they usually clean up fine