I don't think so I have gotten a lot of help from you and the others that have responded. I just needed clarification on your response. Like I said I appreciate the responses greatly. I just needs to know if adding an exhaust will have a negative effect on the performance. Sorry if it seems I like I did read your post but I several times. I am new to small engines. I am used to working on vehicles.
You've changed your parameters a few times in this post, and it makes it very difficult to provide you with opinions.
If you're going to spend the money you don't have (since the Predator you want is not in the cards?) on a VM22, then you've added both fuel and volume to your engine and just like "vehicles" will benefit from a more free flowing exhaust, because the one you have on that GC is a box with a tiny hole. On the other hand, simply adding a free flowing exhaust to the engine wont do a lot except make it louder.
You aren't going to be able to make an intake with sheet steel, from a realistic standpoint. So my point was, you don't seem able to get the engine you really want, I assume because of cost, yet are willing to spend more than that initial cost of a Predator on a far lesser engine as well as some fairly high level fabrication work.
As far as clutch and trail riding, it is obvious that the term "trail riding" means different things to different people. As Michael pointed out, his version of trail riding is far slower than my term. You said flat terrain, so I assumed a 12t clutch to a 72t sprocket at a 6:1 ratio would be okay. However, if you are going to ride in the boondocks, through the mud and rocks, Michael's comment on the 10t being better applies. But no one but you knows what your definition of a trail or trail riding is. You also commented on acceleration, which really doesn't apply to your stated goal of trail riding.
You can swap Max Torque sprockets, but it will involve a new bushing, and since I know nothing of the condition of that clutch, can not offer any more advice on swapping sprockets. Shoe and spring condition, etc. you might have been able to simply rebuild your old one.
Lastly, I had two nearly new GC160's on my shelf that were free. I still have one, very low hours, taking up space. It's value is so low, that in the years it's been sitting on my shelf, I've never considered messing with it. They are essentially throw-away engines, with plastic parts, with no aftermarket support.
My advice to you at this point, is to get the new clutch, get new chain, get it all together and running with a stock engine and see what you end up with. At that point, you will have gained better understanding of small engines and mini bike applications, and how your wants and needs can best be met.
I'm not one of those guys who needs to hear himself talk on the internet, although my massive post count might look otherwise. :laugh: So I apologize for my flippant comment earlier. Over the years, we've had a lot of "ask holes" come in, waste everyone's time, then disappear. Hope you're not one of those guys. When you do get a better direction on where you want to go, the OldMiniBikes Warehouse who sponsors this site has an excellent customer service section, with knowlegable people who can answer just about any question you have. Best of luck.