That is the right jackshaft kit, but it looks like they are out and they won't get in until mid March.
Thank the dock workers on the West coast for that and many other things you can't find.
That is the proper engine type for your project.
It looks like you want to stay under 50cc, wise if you simply can't afford another run in with the law.
I don't know what to think of your $120 motor kit choice though...
Fantastic 49cc 50cc Bicycle Engine Kits for Motorized Bike 2 Stroke Petrol Gas
That is their E-bay product description pic, not mine, jezz...
Looks like a 66cc base with a 48cc head and then the whole thing dunked in a bucket of paint.
Except the silver parts, every nut, bolt and screw look to be painted into permanent place.
Black engines look neat, but you don't paint the whole darn thing into place.
Looking for the best 2-stroke engine kit bargain is usually not the way you want to go when you are going to be spending days attaching more to it, then the combo to the bike to see if it even runs and will continue to.
Personally I like Skyhawks but I just picked up a couple these last week.
http://www.gasbike.net/48cc-black-stallion-2-stroke-bike-motor-kit.html
One is in a bike and it's a nice clean well made engine with good kit parts like a bigger 2.5L tank and some parts you won't use with shift, but parts none the less.
If I'm not mistaken the Yang Dong factory (China Gas) made Black Stallion and Flying House 2-stroke engines as well.
Bottom line is you can spend money riding around, or spend it fixing it wishing you were riding.