Sometimes you can peel off the backing strip on a lcd and fit a backlight, I have done it on a couple of walkman style cd players for a dj, however dont bother doing that, you can get aircraft style instrument illuminators, they are a kind of bolt with a side looking led, you can get them from boy racer type stores too, fit a couple of those and you'll be able to read your temp.
Another thing you could do is hack a couple of these gooseneck usb led lights.
I wouldnt trust metal epoxy, esp if your engine is seawater cooled.