Transmission swap - 6 spd for 5 spd?

[2002celicaGT]

I've got a 2002 celica GT that someone drove a bit hard (the car has 115K miles and the transmission rod broke) and needs a new transmission. It currently has a 5 speed manual in it, and I was considering putting a 6spd in it. Is that doable?

Also, has anyone experience a code 300 / 301 - general misfire/ 1st cylinder misfire on this model vehicle and do you know of a repair? My mechanic opened the cylinder cover and cleaned out that area and replaced the gasket, which was able to clear the code and reset all of the sensors briefly, but this problem keeps coming back. Sometimes the check engine light will even reset itself!

Aside from the above problems, the car has been great since I purchased it at 82k miles. Any general thoughts on this vehicle at this point - is it worth repairing etc?

Thanks!