Any modifications makes the motorcycle illegal unless you have a doctors note.
That includes making it a trike, add a sidecar and everything else, so unless you live in a rural area it might be a hassle...
Personally I live rurally and have an old trike that is a home made conversation... I don't drive fast, it has working breaks and lights, and it's not noisy, so the police still let's me go without a hassle. However, I was thinking about buying an ATV instead... and the same police office who let me drive my totally illegal motorcycle told me that if I get an ATV, then he would have to fine me... because even though my motorcycle is 1,000% illegal, it is still considered a motorcycle.
And again, if you get a doctors certificate, then you are supposed to be able to register an ATV for road use too. Or you can get it classified as farm equipment and drive it the same way as they do with the home made trucks and other unregistered "farm equipment"...