Hello, I just picked up a 1977 DT250. The fuel was being pre mixed when I got it. Well the reservoir is gone, but the pump is still in place. The hoses and cable are still in place also. 

 I am thinking if I rebuild the pump, adjust it as it should be, and purchase and install an oil tank, I should good right? Am I missing something here?

Yes, that idea sounds fine.

Have a look at the parts list on CMSNL or Partzilla and try and get a tank with all of the fitments included - eg the oil level sensor, which links to the red light on your clocks/dash.

I don't know if the ones for your model get brittle or not. Wait for others to confirm and, if they do, take extra care when buying.

Me being lazy, & if the pump has had oil left in it i'd just bleed it up & check it pumps then carry on.

If you do it I would run some premix in it too just to be safe 

If your oil pump has been still connected but without oil for some time , which it likely has been .
It could well have been damaged . the oil being pumped , acts as a lubricant for the moving parts in the pump .
As has been suggested , run a premix whilst determining if the pump will actually work .

As I'm resurrecting old project Yamahas I have had the idea of premixing while forming trust in the injection pump. Most of my projects get sold before I form that trust I'm wondering how to confirm that the injection pump is fun ruining correctly. 
Right now I'm breaking in the top end on my current project, so I I'm mixing my oil ratio @ 20:1 with the reservoir filled but the cable disconnected. I'll be primarily running race fuel that's as close to premium pump gas as I can find, but I'm looking forward to stopping at the gas station to top off with premium pump gas while on a ride if I really need to.

Super easy if running on premix.  

Yamaha manual has output specs for 200 strokes of the pump with pulley marks lined up (idle position) and again for 200 strokes with pulley rotated to full throttle.  

Remove oil line from carb, cap carb fitting.  Start engine and let it idle with oil pump pulley rotated to full throttle.  Measure output at hose while counting 200 strokes of pump plunger.  Compare to specs.  (To measure, I use a syringe with cc markings I got from pharmacy)

Then start over and line up "idle" marks on oil pump pulley and count another 200 while measuring output.  (Plunger barely moves with pump pulley at idle but you should be able to see it bump slightly. Compare to specs.

Many owners I know run some premix in the tank...  use same oil used in autolube...  at a lean mixture like 40:1 while using the pump too.  Theory is if pump fails, premix is a safety net.  Yamaha themselves recommended running 32:1 premix AND the autolube pump for racing.  


 

Just be sure you have at least minimum stroke with the cable disconnected if you're planning to run a lean premix. Or you could run full premix & use the minimum stroke to oil the chain. I did that once on something about 50 years ago. Probably my DT1 & piped the feed line into the sprocket cover so it drip pumped straight down near the sprocket. Not sure why I stopped doing it-- probably made a he'll of a mess haha.