The '07 Agility is a 4t engine. Similar to the QMB 139
The People is a 2t engine similar to the S9.
The big issues are the wiring is totally different, and the engine mounts have to be re-designed.
Cost effective wise it's a winner because the 4t is cheaper in every way. Parts/cost to run. Personally I like the idea, and would do it only if YOU understand the wiring. It's all there you just need to rewire the head/tail lights, and add a new rectifier/CDI to match the 4t stator. or use the 2t stator/flywheel if they fit (I never tried that)
If you go for it I would also consider adding 3" to the engine mount length, and make a spacer for the rear shock of approx. 1 1/4"
This will greatly enhance the ride-ability for adult size riders.