These are fun little conversions- they are my Skorcha Wartrakks, made a little Grot Tanks. I figured only Bad Moons would be so wealthy as to allow grots to have their own tanks. 
The conversions are relatively simple to do. Basically, take the back half of the wartakk model. Since that’s a bit expensive, look around for used ones on ebay. They can be in any condition, because you are just going to cut it up anyhow.
Take the back half and use plastic card to cover the front gap. There is a square bit with a circle turret hole from the trukk kit. That gets put on top, along with a magnet. Then place a battlewagon grot turret into the spot. Add a flamer to the grot turret and you’re good to go.
I haven’t seen too many people field Skorcha Wartrakks in ork armies. I have to say, they are a great addition to the army. First, they are fast vehicles, and create an annoying little unit to harass enemies with. usually opponents are much more concerned with killing the tough stuff in my army. These guys are allowed to drive around doing whatever they want, usually softening up units before trukk boyz pile into them.
consider these looted! I’m giving that a go! I’ll bash one out this weekend coming.
MadGrotDoc has made some similar on the waaagh you should check out, just to give you some ideas for further variants in the future if you ever come to add more to their number