Replacement roof felt material

I have an old wood house with a tiled roof. The previous owner at some point replaced some tiles, but looks like from the inside of the roof. I.e. cutting through the felt. I would like to patch up these holes to make a more watertight roof in case any tiles crack in the future. Any suggestions on suitable materials or methods without having to lift a lot of tiles and likely cause further tile damage?

Wow that sounds a difficult job.

I'm not sure of the 'correct' meathod ... but I guess you could get some thin rubbery material from Clark Rubber and stick it on their with roofing silicone? It's always going to be a little hard to know if you got a good seal or not ... but it may be better than nothing.