I don't know what you mean by "regular"...
If I were building a wildcat, I'd stick with a carbine length gas tube and add an adjustable gas block. You'll be able to adjust the gas accordingly, for your desired loads.
As for custom parts... I'm assuming you're not a gunsmith. Which means that you're going to need one... A really good one. The parts will need to be fabricated from scratch. I won't lie when I say that one-off parts will run about 5x normal cost.
Then you'll have to look at hourly rate. You're not going to find simple "plug and play" parts. You may very well be the first to ever attempt this. A gunsmith will likely spend hours doing a trial and error of parts, trying to get your gun to fire/chamber/eject reliably.
It may behoove you to wait for LWRCi to finish their new 6.8SPC rifle. It has a larger than standard receiver, that uses a new/larger 6.8SPC PMAG. This design is under contract to a foreign military right now, but they already stated that it will become a domestic rifle in the near future, after they (LWRCi) have satisfied their end of the contract. From what I understand, Magpul has agreed to make a shiit ton of these mags to distribute with the rifle. If your google-fu is strong, I'm sure the measurements are out there somewhere.