I have a 50hp Mercury with oil injection and the injector is malfunctioning. Can I just run a gas/oil mixture rather than replacing the injector pump?  

Yes you can.  That was a common problem with the Evinrude and Johnson motors several years ago.  Guys just disconnected it, mixed their own and went on.  I had a couple of them.  Just be sure you are doing the right mixture.

I had an old Black Max 135 years ago.  The local merc shop sold me a $10 part to replace the injection pump/part and told me to take they system off and mix the oil.  I dont remember what the ratio was but the motor ran alot better.    

Not sure what M-Y your merc is but my 2006 90 have a very simple valve that just connects to the throttle linkage so you might want to see about checking that first.  Also understand, if you are getting an alarm for no oil, if yours has the oil tank under the cowling they are bad about the sensors coming loose and causing the alarm, but still be oiling.  

So, if you are basing the no oil on the engine alarm, let me make a suggesting.  Before running it again, premix your gas and oil at a 50:1 ratio in the gas tank.  Make sure there is oil in the oil tank and with the motor trimmed down, mark the oil level with a marker.  The next time you use it, it's going to smoke a lot more because if the oil injection is working, it's going to be double oiling the motor (not going to hurt anything) and when you are done for the day, check the oil level in the oil tank, it the oil is still at the mark you made on the tank, the oil injection is probably not working.  If the oil level is below the mark you made on the tank, the oil injection is working, and you just have a bad sensor.  If it's in the tank as mentioned, they can be glued back with a good grade of CA glue (super glue).

 

You might want to do this for the next couple of trips, marking it each time just to make sure it's a faulty sensor.

 

Just remember, premix the gas in the tank before you do anything else.  It's a whole lot safer to double oil one while trouble shooting it than taking a chance on it really not be oiling.

I replaced the pump about 15 years ago when I didn’t open the oil reservoir vent. The resulting below zero temps pushed the oil through the pump and into the carbs.  It was a pain in the keester to replace then and I was 15 years younger. 

Ask your dealer what is needed to block off the oil flow, buy it and mix your fuel.  When my boat is on the trailer I'll put 3 gallons of gas in the tank then 8 oz of oil and keep doing it until it's full.  That way it'll mix fairly well.