Originally Posted by maxwell 
-remove oil tank
-order a blockoff plate/gasket
-plug some fittings
-create a breather tube
-fill the cavity with oil and bleed
its alot easier to do with the motor out
You can do it this way or you can also get a shorty rotary valve cover like the one from crankshop
45MM NON-INJECTION RV COVERS The Crank Shop Vermont Specializing in Ski Doo Performance Aftermarket Products which eliminates the oil injection and gives you a clean mount.
This will also move the carbs closer to the motor for better throttle response as well, Split Second also makes these, local dealer and won't have to pay brokerage and exchange on the US $$$. You will have to order carb boots to adapt to the plate but let me know if you go this route and I can probably help find the right part # for you from Kimpex (had to go through this myself).
The tube can be connected between the upper and lower oil ports on the back of the motor, fill the tube about 3/4 full and add oil as necessary. Big time oil use (refilling from empty every ride) means the main seals are going. Let me know if you need pics, I have a motor on the bench right now with the same setup that I can take pics of. All you need for a backup is a small 100 ml bottle of oil just in case. Be aware that if you premix and you ride with guys that don't......you won't be able to siphon large amounts of fuel if you run out.
Really simple to do on the 670's. Premix should be 35:1 depending on oil and jetting. I run 35:1 and one range hotter on the plugs (BR8's instead of BR9's).