If you don't have a motorbike license in your home country, you will need to get a local license to legally drive.
People who rent them out might not care at all. But your insurance for sure will if there is an accident. And seeing as Thailand is pretty far up towards the top on traffic accidents... You want to be covered.
This is all relatively easily doable without a agent.
Is there a particular reason why you're wanting to use a agent?
Biggest hurdle would be the bank account.
But if you visit a few branches of whatever bank you're wanting to use, you should be able to find one willing to open accounts for you. But with all the mess with Bangkok Bank, I'd personally avoid those.