YBR series handle engine grunt in a really poor way, as long as it has a powerless engine in its bay it is fine, but you put a power full torqueee engine and it would start rattling and rip aprts its plastic peices.
-> People who have swapped in the past with my assistance or by there own self had strugled with this issue.
-> Also the engine bay is too small (ybr own engine is miniaturized as you might have noticed) to get any thing big shoved inside it and requires re-fabrication and re-enforcement of mounts.
->If you are a resale conscious person and sale bike after use to recover funds for next bike then this is not a good idea, it would be better to buy a paiggio, derbi, wego, united and swap engine in them as they have bolt to bolt cheap upgrades and they are pimped up quickly and can be reveresed completely when ever you want to sell them.
-> If you aer a Die hard YBR lover and you do not have any worries of resale but really want to see YBR cruising like a powerful machine, then do not go for a used engine, buy a brand new one with Counter balancer shaft (vibration free) engine. By doing this you will have piece of mind and power both.