Well, I'm not anywhere near as good with wording as bb, but I'd like to see what I can do about Modernize.
Pretty much all Abilities can be classified into one of two categories:
passive and
active. Sort of like how Poké-Bodies were passive, being constantly in effect, while Poké-Powers had to be activated.
It looks like right now, Modernize is a sort of combination of passive and active. I can't recall offhand any Abilities in the TCG that do this, so let's try and see if we can get it down to one or the other.
Usually, if you pick a Pokémon to give a lasting effect to, there are two ways to do it: placing a marker, or attaching something. If you want to place a marker (which would introduce a new mechanic; not sure if you want to do that or not), the wording would probably look something like this:
Once during your turn
(before your attack), if none of your Pokémon have any Modernize Markers on them, you may put 1 Modernize Marker on 1 of your Pokémon with any Pokémon Tools attached to it. That Pokémon may have up to 2 Pokémon Tool cards attached to it. (If that Pokémon loses that Modernize Marker, discard Pokémon Tool cards attached to this Pokémon until only 1 Pokémon Tool card remains.) [References: Sigilyph PLB]
That's probably not the most accurate, though, as there aren't any cards in the current format that place markers, IIRC. Alternatively, you could attach Porygon-Z to that Pokémon, a la Unown Q:
Once during your turn
(before your attack), you may discard all previous Stage Evolution cards and all cards attached to this Pokémon and attach this Pokémon to 1 of your Pokémon with any Pokémon Tool cards attached to it. As long as this card is attached to a Pokémon, that Pokémon may have up to 2 Pokémon Tool cards attached to it. (If this card is removed from the Pokémon this card is attached to for any reason, discard Pokémon Tool cards attached to that Pokémon until only 1 Pokémon Tool card remains.) [References: Sigilyph PLB, Unown Q]
Both of those sound really convoluted to me, though, so
@bbninjas should definitely take a look at it.