Do I need a smaller or larger sprocket for this?

[zzirrgrizz]

Here is my plan for my moped:

1) The red circles in the pic are where I could possible place the engine.
2) I am going to probably chop off the pedals but leave the gear that's currently hooked onto the right side pedal (or weld it on whatever).

On the opposite side of the existing gear on the pedals I need to know what kind of sprocket I need. I looked at the sticky forum for the parts list of a mini bike but since I will be not running the engine directly to the back tire's sprocket I am wondering if this changes anything. Should I use a large gear for this like the existing one on the pedals or should I use a smaller sprocket like the one in the picture I posted below? I am very new to all this so any help is greatly appreciated guys

 

[JoshTehSquash]

You could probably use the sprockets you got because bmx bikes use 40 pitch chain...
You would need to make a jackshaft of some sort since you need a low ratio for those large rear tires. Use the go kart speed calculator to calculate the gear ratio you would need to go say 15 miles per hour and then put in your tire size and max engine rpm.

 

[JoshTehSquash]

Also you should probably put all your questions in one thread so that people can see all of them in the same place

 

[landuse]

A engine might be too much for bicycle sprockets