12g Wingmaster can I shoot 3"? & part # for better firing pin?

[Synchronizor]

There are three parts that differentiate 2.75" and 3" 12ga 870s; the barrel, the ejector, and the ejector spring. The changes to the ejector & ejector spring are just to help the longer shells clear the front of the ejection port; they're not critical to actual firing or safety. With just a 3"-chambered barrel, the gun will cycle, feed, and fire 3" shells just fine, but ejection of the spent hull may not be very reliable.

The magnum ejector & ejector spring are common replacement parts, and should not be too difficult to track down. Cabelas isn't a great source for small gun parts; I would check the big gunsmithing retailers like Brownells, Numrich, or Midway to see if they can deliver to you. Remington will also mail these parts out via phone, internet, or mail order, but thy may not be able to do so internationally. If none of that works, check with gunsmiths in your area. 870s are super common, so they will likely know how to get factory replacement parts. You may want to let a gunsmith do the ejector & ejector spring change for you anyway, it takes a bit of know-how to do it properly.

As for the firing pin, what exactly are you looking for? There aren't really any better or worse versions of the firing pin, all 870s use the same part, though there have been some very minor tweaks to the pin over the years. If you're simply wanting to make sure you have an up-to-date pin with no wear, just order the current standard firing pin part F17436.