Now your just being a cheeky ratbag. Alright, fine, let's talk Da Vinci.
In Fate, Da Vinci claims the title of Renaissance Man (notably, something he didn't actually have in life and failed to actually accomplish, more on that later), boasting the ability to make basically anything given time and materials and a vast knowledge pool. This goes so far as to extend to his Noble Phantasm, which allows him to analyse a Noble Phantasm and return it back on the user, countered.
My first question. What there did I just list that the combination of Arturia and Muramasa *can't do*? Da Vinci immediately smashes headfirst into the drawback of fulfilling the same niche as your main characters, and is arguably worse at it then they are, since Arturia and Muramasa are designed to be working together at basically all times, to the point where Muramasa has traits specifically to help his partner.
So now that we've got the Fate side of the argument out of the way, namely that stats wise he doesn't actually bring anything to the table, lets talk history.
Da Vinci was taken advantage of his whole life and was notoriously bad at managing funds and resources. You better believe that'd find a way to be represented on his sheet. There's also the fact that he's the number one neet in existence and hates people in general, but that kinda comes secondary. Just shoving him in a workshop doesn't change that he's almost certainly gonna be a hog on your resources.
Which brings us, finally, to the one thing he does offer. Technology. Except you are in Bretannia, where you don't need anything more complex then gunpowder because no one will use it, and the peasantry isn't going to be able to learn to drive a tank competently in the time you've got.
Those are all the arguments against that come to mind. Lets talk about the big elephants in the room.
Mona Lisa. Da Vinci can make puppets to inhabit, which means he ain't dying. Its an unconventional method of immortality, but it works. Then there's the other big elephant, names that Da Vinci is a confirmed magus and probably has a good understanding of every magic that existed in the west in the Renaissance.
I just don't, personally, believe those two points outweigh the gaping 'everything Da Vinci offers, Muramasa already gives you' problem I see.
But that's not on me. That's your decision.