1) You don't need a Texas title, you need a Canadian title, assuming you are documenting the boat in Canada like SD and others suggest.
2) Allanbc says you have 20 working days after sale completes to leave Texas or else the sales tax is due. Some states do not count time spent on the hard with work being performed, some do. Check with Texas officials, and good luck getting an accurate answer.
3) You could delay the close of the sale until after the work is completed. You'll need a very tight agreement with the seller to protect you both. Someone, presumably you, will be paying for the work done, and you'll want to make sure the seller can't change his mind and not close on the deal. But if you do this, your 20 days won't start until the end of the work period.
Texas is pretty tough to work with. I have had problems on almost every transaction with them.
Here is a link to the boat registration information. TPWD: Boating - Boat Ownership
"When in command, command." -- Admiral Nimitz
Difference between a power boater and a sailor out on the water: A power boater is going some place special, a sailor is already there.
s/v Zotz 1981 Pearson 365 Ketch Hull #375