Normally, switching an engine isn't all that complicated, but there could be a few issues involved in your particular situation because your minibike was sold by an American company, but the bike was made in China. There have been complications trying to fit other parts on and in some of Baja Motorsports other models because the Chinese don't follow American standards for things like engines, chains and sprockets. Worst case scenario would be you needing an offset plate or redrilling to mount your new engine because most minibike engine mounting plates have slotted holes to allow you to move the engine forward or backward to adjust the chain tension, but there is no side-to-side movement to allow for adjusting chain alignment. Sometimes, when installing a larger engine, there can be chain alignment problems because the new engine is wider, and the clutch position on the crankshaft is farther from the centerline of the mounting base of the engine. Your bike may also have metric standard chain and sprockets on it now, and they would not be compatible with a new American standard clutch designed to fit your new bigger engine. We have piles (literally) of Chinese minibike parts in our shop, waiting to be machined to fit American standard minibikes.
The Tecumseh engine Rupparts recommended is an excellent choice for your project because it is specifically made for minibikes, and already has a throttle hookup made to fit the throttle on your bike. It will save you the problems associated with converting a fixed-type throttle found on most other engines. Rupparts is a dealer for lots of minibike parts, and I think he sells that engine for about $200 plus shipping from Missouri. He might also have clutches, and chain and sprockets if you need to change yours to American standard. The Tecumseh Powersport line of engines also come in higher horsepower ratings, each one costing a bit more as the horsepower goes up. Most minibikes function well with an engine between 4 and 6.5 horsepower. 3/4" is the standard diameter for crankshafts in that horsepower range, and clutches and sprockets with a #35 chain pitch work well. All commercial-use engines have a counter-clockwise rotation of the crankshaft, and that is standard for minibike usage. If you buy a used engine, it may be an Industrial-use model with clockwise rotation...check it by pulling the starter and watching the crank...there are a few out there.
American standard clutches atttach to the crankshaft by one or two small allen head set screws in the hub of the clutch, and have a built-in 3/16" key to prevent slipping on the shaft. They should also be held in place by a 5/16" x 3/4" fine thread bolt with a lock washer and a flat washer at least one inch in diameter. This bolt threads into the end of the crankshaft, and keeps the clutch from moving outwards along the crankshaft, causing the shaft to bend or break. It's just one bolt, but it's the most important bolt on a minibike engine.
If your chain is #35 pitch, or your clutch is 3/4'' diameter and you can use it on your new engine, the rest of your drivetrain including brakes shouldn't need to be changed. If the phone book lists a dealer of engines in your area, they should be able to test a #35 drive sprocket of any diameter they have available against your chain and tell you if it's correct. If they just look at it and say yes,...don't use that information...many metric chain pitches are close enough to appear the same as American standard, but they don't function properly when parts are mixed.
You're the first person I know of to try an engine swap on this new model...it should be interesting. Keep us posted as you progress, someone else may benefit from your first-hand knowledge. If you want to ask anything else along the way, that's ok too.