OBD2 Codes...CAT, O2 Sensor, or Something Else?

usually most P0300 codes no matter the make or model of vehicles seem to be a misfire issue? or at least that is what i have noticed...

What does that mean? Timing? Spark Plugs?

336 and 340/341 are crankshaft position and camshaft position sensors circuit issues, respectively. 420 could be an exhaust leak, causing the Cat sensors to misread, or just an issue caused by the other codes. The 31 code is kind of confusing, it is either an ABS issue, or if it is supposed to read 301 it is a misfire in cylinder #1. You may try to contact a dealer and see if the vehicle ever had any recalls performed, and if there are any updated reprograms for the ECM. From the sounds of it, though, there may be a computer issue for it to be throwing all of these codes at the same time. I would recommend checking your wires for loose connections, rub-throughs, corrosion, etc. Especially if you have not had any work performed recently.

timing belt

Hmmm.... Does it restart and run normally? If not, sure: I'd indeed check the t-belt, as noted above. Note that misfire codes are PO301-4. The crank/cam shaft sensor codes hint toward the t-belt, rather than wiring issues. The PO420 is just your cat getting old, bad gas, or the electrical fairies that plague Subies and Toyotas with this code. BTW, I would NOT ignore the knock sensor, as they're cheap (same as Nissan), and will contribute to smoother running, engine protection, and improved power and fuel eco. Good luck.