A blitza bomber might serve you better than a dakkajet vs necrons. Armorbane on the bombs can help you crack their vehicle shields. I agree with above about lootas; the only thing they'd really hurt are warriors but even then they can still reanimate. ap3 and tank hunters are more reliable.
Trukk boyz aren't terrible in medium size game, they just require 3 or 4 trukks so they can get up the board. If you decide to use more bikers, watch out for triarch stalkers and tomb blades with nebulascopes (both ignore cover). Grot riggers won't help much since they can glance trukks to death with their infantry. 'eavy armor will only matter in close combat most necron weapons are ap4 or better. A warboss on a bike with a painboy on a bike with survive a lot longer.
I'm not sure what models you own but from my models I would use :
Ork Horde Detachment:
Warboss, klaw, stikk, bike, bosspole 140
Painboy, bike 75
Painboy 50 (for meganobz)
10 boyz, nob upgrade, klaw, trukk, rokkit, reinforced ram 130x3=390
3 meganobz, trukk, rokkit, reinforced ram 155
5 tankbusta, trukk, rokkit, reinforced ram 100x2=200
5 warbikers, nob upgrade, klaw 105
blitza bomber 135
If you don't have enough trukks then drop boyz for grots and add more to the biker and tankbustas unit sizes