Post a WTB in the classifieds. You can also find them on e-bay. Just make sure you buy from a reputable person. You are looking for a CPUNXT2 mpu, and you are going to want to make sure you get the entire thing with the cover attached. Some dealers just sell the mpu without a cover, but a cover is important because of air circulation across the video card.
The 4:3 aspect ratio games are typically the ones that are G+ type games. If in doubt, you can post a question here and somebody will be able to tell you. Basically any bb1 compatible bb2 game is 4:3 aspect ratio.