The bike idles but stalls as soon as I put it into gear

One of the most common failures that can cause this problem that I have seen before is the Torque converter solenoid (TCC solenoid). This solenoid can stay stuck on and cause the torque converter to stay locked up, this will cause the vehicle to stall whenever it is put into gear. The vehicle will start and run fine in park or neutral but will stall out as soon as it is put into gear. This is the most common failure that can cause this.
Also a malfunctioning idle air control valve can cause this, if the valve isn't moving or responding to bump up the idle when put in gear it may cause the vehicle to stall out, the computer commands this valve to allow more air in to bump up the idle to compensate the load on theenginewhen put in gear, if it isn't moving or isnt moving fast enough the idle may be too low and could cause the engine to stall out.Have also seen bad crankshaft position sensor cause this ,as well as mass airflow sensor's ,TPS - throttle position sensor's . Your best bet , take it to a qualified repair an have it diagnosed . No guess work .

Doubt it's the O2 sensor.More likely an issue with your IAC(Idle Air Control) motor.Quite often they get plugged up with carbon and stick in position.

Only thought Is the Kick stand switch
Is a kickstand on or was there one ever on this bike? If the switch thinks the kickstand is down it will stall out as soon as you kick it into gear.

2506 PCM/ECM Power input signal range/performance
110 IAT Intake Air Temp sensor 1 circuit
104 MAP/ Baro sensor low input
You probably did do some damage by not isolating the engine and controls out before the repairs/mods were started.

when was oil last changed sounds like clutch plates sticking

with the engine off put it in gear pull in the clutch if you can't move the bike the clutch is dragging and needs adjusting

There is a safety switch on the kick stand of that bike that keeps it from running while in gear. It sounds like that switch is bad and killing the bike as soon as you put it in gear.

clutch is either adjusted wrong or it time to replace it sounds like its not disengaging