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Then you just end up with shitty looking concrete tiles...or you stump up for the correct clay tiles and have a humongous bill. You'll have a neat roof though, I guess, which would then be totally at odds with the ramshackle nature of the rest of the frontage, and then you can't stop and....
If it is just a few cracked / dislodged tiles and there is no evidence of leaking or rot, then you can replace the tiles with reclaims and pray to the roof gods for mercy :)
Once you start touching the roof you may as well replace the entire thing. I’d factor in a new roof cost.