If you are looking to do a engine swap on your Yerf Dog go with the lastest technology engine. You'll need this adapter kit - click here.   This is a GY6 engine that mount much like your current engine.  Comes with:

• Built in reverse -  now you'll be able to drive in reverse and not have to get out and lift the front of your buggy.
• CVT Transmission
• Can change roller weight to fit your riding conditions
• Can install some performance transmission parts

DO NOT DO A PREDATOR SWAP ON YOUR YERF DOG!  WHY??

• It will cost you $1000 to $1500 by the time you are done.  Sure the engine is cheap but look at the entire swap before you get started.
• The Predator lawnmower engine is not designed to run over 3000 RPMs.  You'll have to spend a lot of money to modify the engine to run at 7,000 to 10,000 RPM for a go-kart
• The GY6 engine uses modern CVT transmission technology.  All of the high end ATV's, side-by-sides use CVT transmission technology because transfers the power from the engine to the wheels much more efficiently.  The Predator engine uses 1980 torque converter technology.  
• You'll be replacing belts each month.  Your belt operates as a clutch continually slipping while at idle.  Again old technology.
• It is a dangerous engine.  You run the risk of ripping the hair/scalp off of some little girl or, anyone with long hair, as has happened to hundreds of people - see photos.
• The torque converter is jerky on acceleration and the engine and loud and clunky.  With the Predator engine you have 1940 engine technology and 1980 transmission technology.  If you don't mind antique technology then go for it.  And since you are going backwards why not consider a steam engine.  And now since you have a fast lawnmower why not put some blades on it and you can cut the grass while riding your go-kart
• You will not have reverse with the Predator engine.

The GY6 engine comes stock with a 67mm big bore cylinder and piston and an 8.0mm high performance stroker crank. 

You will need to relocate your axle sprocket hub over by putting it on a lathe, cutting the weld, moving to new location and rewelding or you need the BDX axle swap kit.  

Specifications:

• The engine in this listing is an actually 232cc engine (no rounding).  Others list this as a 250cc engine.  Comes stock with a 67mm big bore cylinder and piston and 8 mm stroker crank.
• Comes with a 18 pole stator, compared to your stock engine 8 pole stator, for more power for lights and other accessories.
• Oil cooled engine