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.
Never listen to someone describe why your project will not work unless they can show you the broken pieces of their own version. - Robert Gainer