rebman, as Buick said, that hole is supposed to be there. No need to take it in as long as you don't mind a few hours of (relatively) easy work. Unless your bike is an '07, in which case you should call a dealer with your VIN. The '07s had a recall about this and they may fix it for you for free.
OTHERWISE, do a google search for "cmx250 service manual" and use the scribd (no relation) link. Follow pages 10-1 through 10-3 to remove the cover and fly wheel. The hardest part in all this is getting that flywheel off. I highly recommend using Autozone's loan-a-tool service for this. Tell them you want the "5-ton jaw puller"
Great Neck/10 in. jaw 5 Ton puller (2707
| Jaw Attachment Puller | AutoZone.com, you'll have to leave like a $50 fully refundable deposit. You need the 5-ton, the 3-ton is too small, I had to go back and swap them. Also, if you don't already have one, buy a rubber strap wrench
OEM/2 pcs. aluminum body strap wrench (25376) | Oil Filter Wrench | AutoZone.com Don't forget to buy the wrench, I wasted a good amount of time trying other things because I forgot it. You're going to use the strap wrench in place of the "flywheel holder" in the manual, and the jaw puller in place of the "rotor puller". You don't actually have to remove the stator, you can just let the case cover hang to side if you're careful about it. Even with the strap wrench holding the flywheel in place, you may still need a "leverage enhancer" to break that nut lose. You may need to wrap the strap wrench around the jaws of the puller to hold the jaws together on that step, as the back of the flywheel is curved and the jaws want to slip off. Remove the stator driven sprocket.
You will probably have a black seal come off the shaft behind the sprocket, note which direction it faces, because if I recall it will only go back in properly one way. That little guy is the source of all our trouble. Give it a cleaning and inspect it. It's probably in good shape and just popped out (rather than the seal is shot and tore out). Push it back on and into its hole. You should feel it snap in and then be unable to (easily) get it back out. Pick back up on page 10-8 to reinstall everything. As long as you didn't completely destroy the paper gasket when removing the case cover you won't need to replace it, a few tears are probably ok as long as it all fits back together when you put the case back on. The gasket is for vibration dampening/spacing, it's not there to seal oil in. Careful with the stator cables. Now's also a good time to clean the gunk off the front chain sprocket area. Mine had so much crud from the PO that cleaning my chain was useless because it just ran through more crud.
Next, think about why it happened to begin with. The usual suspects are air filter, full puke tube, and over filling oil (me). Check the air filter and puke tube, and if they are good and clear then it was probably oil. Of course, checking your oil now won't tell you that, you lost so much. Correct is 1.6 qts. I now put in about 1.5 qts, just to get it about midline of the cross hatch, just because I'm now so worried about over filling it. Also, correct oil checking is to run the bike a few minutes, cut it off, rest (don't screw it in) the gauge on the threads (after wiping it clean of course) and level the bike for like 30 seconds.
I guess I really should take pictures if I ever have to do this again.
dragon, you keep saying gasket. Your bf is right that it almost certainly isn't the gaskets, they aren't sealing any oil. I'm not sure about the starter driven seal, but it's worth a shot if you've already checked the stater driven seal, assuming of course that you didn't add an r and meant stater to begin with, in which case, yes, that seal =P
I hope that helped at least one of you guys.