My bike has been running good for a while with regular changings of spark plugs and good care, however, I've got two questions that I need answered. Though i'm quite happy with my bike, I need a little more oomph, i've been contemplating about buying a 300-400cc engine and installing it, it has a relatively large body that i think would hold the engine, would that work?

Pics:
http://imgur.com/ltqLGFM,eZjyI5C



Secondly, spark plugs go bad every month or two, am I doing something wrong?

Thank you in advance.