So are you asking about the two different size 2 strokes because the smaller one is being carried as a spare engine?
Other things would be using 2" wide road tires, front suspension and a suspension seat post. This will give you a much more comfortable ride. Comfort is a must when riding long distances.
You should also use double walled rims and disc brakes. They come in very handy when having to bump brake going down steep hills especially with a 200 lbs load behind you.
With the shift kit use a cassette hub instead of a freewheel hub. The FW hubs axles are prone to bending. As for the cassette use a 8 speed 34-13. I'd set the high limiting screw on the rear derailleur to exclude the 13t sprocket. This'll then be the same as having a 7 speed 34-15 cassette.
I'd contact David Staton of Staton Inc about one of his shift kits. With the SI shift kit it uses a double freewheel system. It can hold 3 chainrings instead of one unlike the Sick Bike Parts shift kit. In my SI shift kit I had double White Industries Trails freewheels installed. It comes with a 28t chain ring and I'd suggest adding a 36t and 44t steel 4 bolt chainrings. The SI SK engine drive sprocket is a 44t.
This setup gives you a 3x7 drive system. The shift pattern will work like this: 1(1-3), 2(3-5), 3(5-7). Hill climbing 1(1-3); level ground cruising 2(3-5); down hill or level high speeds 3(5-7). Center the wheel to where 2(4) puts the chain in a straight alignment. With this set up the lateral stress on the chain will be greatly reduced thus reducing chain failure. Get a good chain like a Sram 870.
The shifters put them on the left handlebar. Use a 7 speed twist shifter for the cassette and a thumb shifter for the chainrings. A little trick I did on my late brother's build was was to engrave a line down beside the shifter numbers on the twist shifter and engraved a dot beside the numbers on the thumb shifter. Then I painted the dot beside CR1 red, CR2 yellow and CR3 green. Next on the line on the twist shifter paint (1-3) red, (3-5) yellow and (5-7) green. This way you'll keep the red rear cogs on the red front chainring, the yellow sprockets on the yellow chainring and the green sprockets on the green chainring.
Get a techometer/hour counter then be in a gear where the rpm stays around 6000 at full throttle. Use starting gears; be in 1(1) before stopping on stops going up hill. All other stops use 2(3).
Finally for the engine get a pull start. You don't want to bump start trying to pull a load. This'll also mean installing a centrifugal clutch. Then there's a few options on how to get the primary reduction going into the shift kit. I personally suggest using a double jackshaft system. In your 1(1) 28:34 you'll need a reduction of at least 60:1. A single jack shaft means having to use a small sprocket 9t and a large sprocket 52t. This can put a lot of strain on the small sprocket. Then a 15t sprocket to drive the 44t shift kit sprocket.
If you want to use a double jackshaft system let me know.
