I think you should just make it so that you only get a limited number of kills in any particular airplane. Once you hit your quota in say a tempdoraspix16u4-3 its off limits for the rest of the tour.
I like that idea. The kill limit could be
10 x ENY rating, even if ENY was disabled.
An alternative to this idea would be to have a limit on an ENY rating, instead of individual models.
ENY x 10 = limit, so:
50 kill limit in 5 ENY rides
100 kill limit in 10 ENY rides
150 kill limit in 15 ENY rides
etc.
There would need to be snowflake rooms with crayons and puppies if this ever came to pass.