Well I have a simple cost effective solution that is a lot of fun and also addictive.
I recently purchased a few Flying Pigeon bicycles from China and I have motorized one.
Standard Flying Pigeon cost R1300.00 incl VAT excludes delivery. Install a 49cc or 60cc two stroke engine and you can get 50+ kms p/hour out of this baby. Braking still interesting

Powered Flying Pigeon my total build cost is approx. R 3500.00 Bike & Motor incl VAT excludes delivery.
Just needed to make a small bracket and extend exhaust.
. Currently working on a Board Track Racer. This is great fun and another hobby for us aircooled nuts.
I have some new bikes in the box and the engines you buy on line ZA supplied and delivered in 2 days.
Regards
Madbones