I'm sure any standard stainless bolt and nut will work if you can't get to the dealer right away. Putting in a longer bolt with either a hex or Allen head with double nuts on the bottom might also help. In addition to the big washers I also tie the loose end of the line around the taught line with a clove hitch knot then wrap the excess around like a hang mans noose ( unfortunately the only two knots I know lol). I carry long nose pliers and extra rudder line in my tool kit.
Hopefully this will keep you going and get you by until Hobie can address the problem.
I'm on my third TI now, and find those rudder lines to be the thing I have to adjust or have problems with more than anything else on the TI. Because of previous problems with my earlier boats (having to adjust the rudder lines constantly), as soon as I got my 2012 TI (even before sailing it) I used Tonystott's washer trick, I removed the screw, wrapped the loose line around the washer on one side, put the screw back in partially, pulled the line tight, then wrapped the line around the screw and tightened it down. This gives you two extra turning corners that the line has to slip past (now the rudder line is pulling on the washer against the side of the screw rather than the screw threads). This seems to hold a little better and less frequent adjustments are needed. When you do need to adjust you simply loosen the screw unwrap from the screw, pull it tight again, then re-wrap around the screw and tighten again. I then re-tie the clove hitch and hangmans noose knots. I know it sounds like a pain, but it seems to work (I have to adjust the rudder lines a lot less than I used to, plus the lines don't get all cut up like they used to on the screw threads. This rudder line joint is probably the highest stress point on the whole boat. You no longer need a gorilla to help tighten the screws since your no longer depending on the screw tightness.
Hope this helps
If the hex socket (where the nut goes in the plastic) is buggered up, it might be worthwhile removing the rudder flip it upside down and pour marine epoxy around the nut to keep it from spinning), actually will be stronger than original setup when your done. If you do the washer trick, make sure there are no razor sharp edges on the washer (defeats the purpose).