I once posted some interesting finds on how to read Middle Welsh here:
But, as far as I can see, both reading guides give examples of Middle Welsh in modern spelling, while if you try to read the books themselves, they are mostly in the Middle Welsh spelling. But apart from that, people who are more advanced than I am have told me that the grammar is rather similar, and some vocabulary hasn’t changed much, so if you can understand the modern formal language, you won’t have much difficulty with Middle Welsh.