Anything you do there will be very temporary at best. Especially if the actual problem is corrosion and very thin metal.

Using any epoxy from the outside really isnt going to be great.

How much clearance do you have at the bottom of the liner ? Access ?

Could you machine up a press fit ring to press into the bottom of the bore and use a retaining compound to seal it ? ( ie unbolt rod and slide this up )

Or maybe braze a patch over the hole ?

Either way...it wont last indefinitely

Or if you can access the actual waterway near the bottom of the liner, fully flush/clean the block and fill over the problem area from the inside with epoxy cement
The yanks fill a lot of race blocks with cement to strengthen them, so same idea really. Although I'd think cleanliness is important. ( not actual concrete type cement obviously )