Degassed tube how to fix? repair?

Do you mean de-purged tube or de-gause tube? Image intensifiers are not like CRT where they have a gausing problem. If a spot is burnt into the image intensifier, it unfortuantely is permanent.

If you mean de-purged (gasless), this can be done by a reputable service/repair workshop with nitrogen.

You mean your image intensifier tube lost its vacuum? Toss it. It’s a gen1 tube. It’ll cost three times the value of the scope to repair it.

I dont think there is a way to repair a degassed tube. Generally once they ar compramised that is it for them.