How to title a motorbike that has no title or bill of sale?

I have a Honda CT70 that was purchased at an auction many years ago. I do not have a bill of sale and the bike was never titled because it was never street legal. I would like to restore it and get it titled to ride around town. Any help is appreciated. And I live in Wisconsin as well.

Ask in a different section - Cars & Transportation - insurance & Registration
The people that answer there work at the DMV
If they don't give you a reasonable answer, there's a way.
Companies can put a mechanics lean on it.
That means the owner never paid for repairs and the repairman takes ownership to cover his costs.
Google - mechanics lien.
You have to be a licensed repair shop.
Some places will take care of the paperwork for a fee.

You have to ask the WI DMV how to do it. You might have to pretend that you assembled the motorcycle from a bunch of pieces that you collected over the years.

Don't tell them that you bought it at an auction without a title. It's not a lawful way to sell or purchase. While a motor vehicle may not be street legal, and is unregisterable, it always has a title.

You mean it was never "registered" (plated) because it was not street legal. The original purchaser most certainly had a title.

What if you provide the VIN (serial number) from the frame (it will be stamped somewhere) and they find it on a list of stolen vehicles from "many years ago"?

If your state won't allow you to title it as an "assembled" motorcycle, you might have to go to another state that will, and enlust the help of a local, to get a title. Then the out-of-state title could be transferred to you, and you transfer the title to yourself in WI.