how much of a pain will this be.

Going to the ocean to do some redfishing. I want to put a small trolling motor on my kayak so I can get to spots quicker. So that makes it a boat.

I bought the kayak two years ago. I have no receipt. The serial number is on the boat. How difficult is this going to be?

I believe this is what you need, along with the vessel registration application.

http://tpwd.texas.gov/publications/pwdfo...ntification.pdf

And a little bit of cash..

It should be easy. I bought a kayak Thursday and they said no paper work is involved anymore.

What Cigga posted should do it. I have a TM on my Commander, and it was almost painless. Take a clear photo of the serial # with your phone and bring it with you.

I added a TM to mine here in Texas and registered it in one day. Got my lettering and nav lights on all the time and I am all set. Remember that it will be a "boat" and requires all the same safety features as a boat minus the fire extinguisher unless you put a small outboard instead.

How do you mount the TM on the kayak

What kind of yak you have? I have an Ascend FS128T and there are you tube videos on how to mount one. I also have a Hobie PA 14 but no TM. I could buy a pre made attachment for TM made for hobies but I have no need since I can peddle.

The serial number is far more important that bill of sale, but that would be nice to have also.
If you purchased the kayak from a store, give them a call--they will have a copy of the receipt

Take a picture of the serial number and a picture of a pencil on paper rubbing of the number. Fill out the form linked above, and you should be good to go.

If you can't get a receipt or bill of sale make one and have a friend sign it saying he sold it to you for a set price. You will have to pay taxes on the price you pay for the yak. Not trying to rip the state off but having to register a kayak with a TM is useless in my opinion. You can make the bill of sale out for $50 and have him sign it saying you bought it from him used. Should save you a few $ on taxes.