Just some important notes and my opinion from my 1st hand experience with 40+ wheel gear shifter builds with 2 and 4 stroke engines...
Your first consideration is what parts you have on hand and your fabrication skills.
I build higher end custom motorized bicycles for a living and one thing I know is I find it better to get a 'box of parts' as a kit with everything that is known to work well together and go from there kind of like a box of lego's or tinker toys.
In short a 2HP 49cc 4-stroke HS kit from gasbike.net with the nice new Grubee 4G cent clutch transfer case and everything you need to build a direct drive is ~$350, $50 less than just a Q-matic transfer case for any Pred bare engine with no parts to mount it or make it function.
Then there is the Sick Bikes 4-stroke 'box of parts' kit for $200 which leaves you spending $150 more than just the Pred and Q but not only do you have everything you need to use your bikes back wheel gears with the engine you have a bunch of spare 4-stroke engine mounting hardware and drive system parts like the back sprocket and rag joint, mounting plate, and hardware for future use which as a builder I will use eventually like I did for the Pred build.
So far it is a toss up in my mind for pros and cons as which drive is best as I like both but again it all comes down to is your time, skills, budget and parts on hand.
For example on these two 4-stroke builds I had all the the leftover parts which the SBP kit which replaces most everything but the needed parts like the fuel and control system parts.
In short about $500 total parts for a legal in most states 4-stroke shifter with nothing else to buy or have to fab, especially the mounting, exhaust and throttle stuff for a Pred.
Again in my case with a bunch of extra parts from most everything 'parts in a box' kits and build a lot of bikes I am a in different category as most builders, but if you want a nice reliable 4-stroke that will run with a 79cc Pred but be able to ride at 5 MPH to ~38 MPH it's a hard system to beat in my build book.