Given you want a mechanised force you may wish to check out the speed freaks list as that doesn't need gretchin! You can then sell/swap the gretchin and other units for more speed freek stuff!

A good combined document for ork army lists is here
http://epic-uk.co.uk/lists/Epic-UK Ork Codex.pdf
You might be able to do a 1 for 1 swap with others for the gretchin with your madboyz, wildboyz and stormboyz as other lists like them (and they are otherwise unavailable).
Otherwise pm evil and chaos as he might have a fair few gretchin laying around.
However, you could just make them up as well as use them as gretchin. It would be clear they aren't normal boyz. In your ork army clearly the wildness, disiplin or madness makes them expendable just like gretchin

So, given that you could go and buy another box of infantry on ebay, or collection of old orks on ebay, which makes use of all those lovely banners and warbosses etc that you can't get anymore (in fact do this!

Then come back for more advice

). But assuming you don't heres how I would make it up
'Nobz' - 10 stands total
1 stand of 4 warbosses and 1 banner - Your supreme commander
6 stands of 1 warboss, 1 banner, 1 heavy weapon, 2 nobz
2 sands of 1 banner, 3 Nobz, 1 heavy weapon
1 stand of 1 banner, 2 nobz, 1 heavy weapon, 1 boy
'Warbikes' - 10 stands
Again someone would be happy to swap boarboyz for warbikes no doubt - but until them make them all up and use them all as warbikes.
10x Ork Boarboyz - 2 per base, 5 units
10x Warbikes - 2 per base, 5 units
'Gretchin' - '17' stands
Proper Grotz
5 bases - 1 boy (the runtherd), 4 grotz
Then make up the stormboyz, wildboyz and madboyz as more grotz as needed - 12 stands - Again though you can probably swap them!
10x war Buggies - 10 war buggies

10x Tractor Beams - 10 big guns
Covert 2 (add bigger gun etc) into oddboys. I prefer zaps for infantry.
Boyz
154x Ork Boyz
31x Orks with Heavy Weapons
Make as many as you like

4 boyz and 1 heavy weapon is good
So all that suggests for an infantry force
'Big' boyz mob 350
4 Nobz
12 Ork boyz
4 grotz
'Big' boyz mob 350
4 Nobz
12 Ork boyz
4 grotz
Could mechanise these chaps with the gunfortresses, or with battle and gun wagons. If not mechanised worth considering if you wish to add extra boyz and grotz to them. Otherwise add things like both the stompas and a flak wagon.
Normal boyz mob 200 + 50
2 nobz
6 ork boyz
4 grotz
+ 2 ork boyz, 2 grotz
You can mechanise them for 140 points with 4 battlewagons, if doing that I would throw in a couple of flakwagons as well. Or you could use the gunfortresses. They also fit neatly into a lander!
big guns mob 175
4 tractor beams
1 supa zap
big guns mob 175
4 tractor beams
1 supa zap
Kult of speed 275
10 warbikes
1 buggie
Kult of speed 225
9 buggies
Big blitz brigade 350
Now the 'ideal' for me compostition is a ratio of 1 oddboy, 1 gunwagon and 2 flak. however you haven't got much flak. So...
2 oddboy supa guns
2 flakwagons
4 gunwagons
Normal blitz brigade 200
1 oddboy zap gun
2 gunwagons
1 flakwagon
Normal blitz brigade 200
1 oddboy zap gun
2 gunwagons
1 flakwagon
Fighta squadron - 200
4 fightas/fighta bombers
All that is 2750 points.
Add in gunfortresses to mechanise something and you can spend another 500 points (125 each). Alternatively use a bunch of gunwagons and battlewagons to machanise a big mob.
You do have to make one of the 350 point formations bigger to avoid having more than 1 BTS, adding stompa or mechanising one of the big mobs would seem to be the answer there. Or even just adding some boyz and grotz. To make a 3000 point force you would most easily add battlewagons to a big mob for 280 and cut 30 points elsewhere (ok not that hard) or get rid 125 points of extras (the extra kult of speed units and 1 boy + grot) to add 3 gunwagons to the small boyz formation. Or just have another gunwagon mob with some extra boyz and grotz for one of the infantry hordes.
For the future - stompa mob with supa stompa and maybe some dreds! As well as everything else
