hey there, im new to the forum too, but ive been round off road two strokes for a while, so heres my 2 cents...
I wouldnt worry about the oil pump too much, they are generally reliable. But, all it does is give your engine the oil it needs, and you could give it a test.. i dont have it on me, but I have the yamaha service manual for all blasters n i can try and email it to you if you want.
You can disconnect the oil pump without removing it if you want? If i remember right, you dont even have to remove the right side of the engine to take the gear out or anything.. (disregard that, its another story)
I dont know what others would say about this, but you can disconnect the inlet tube to the oil pump, and then block the hose, and the inlet using a bit of tube that you put on and then seal by bending it and wrapping a bit of wire on it. Then, take the outlet tube off the pump side and MAKE SURE YOU BLOCK THIS OFF SECURELY! like bending the end, and wrapping it in wire, and leaving it in under the oil pump cover so it dosent move around. Be sure this is sealed off as it would allow unmetered air to enter the engine and make your mixture lean.. Then before you do anything else, remove the gas in the tank, and measure it, and mix in 33 cc's of oil per liter.. a 1:33 ratio in other words.. fill ur tank up with that, and test it out.. This will make your quad run a tad lean, as the oil will take the place of some of the gas and will make the gas slightly more viscous (thick).. its ok in a pinch, but if your not comfortable with fooling about, then HOW DO I SEND HIM A SERVICE MANUAL FILE THATS 30 MB? Because theres a bonifed yamaha procedure to measure the output of the pump.. if i cant send u the whole manual, Ill find the pages, n copy n paste them in..