I have my registration for my SP140. The builder is OpenPPG, not yourself.
As a serial number, since they don’t provide one with the unit, you can create your own, that’s what I did. You should check first if it is already in use with this TC search engine
Max take off weight is an information you can find on your wing, it’s not about the unit.
Let me know if you need more info, I can go in more details if needed.
Then fill out the form from this website, include copy of your payment for registration, copy of passport, picture of data plate on your sp140. I bought data plate from aircraft spruce and just used label maker with clear tape to put the info on the data plate. Will get it engraved later. Finally mail it all to TC, mailing address is on the application form. You can also buy registration marks, or if you don’t TC will make them up for you
It’s deemed a basic ultralight, I used 130kg max payload as that is my gliders max weight, I added my own item when it asked about landing gear and just wrote foot launch. I made up a serial number.