i restored and modified a HH mini shark for my daughter's 6th bd. Originally it was a 50 cc, which was long gone with a lot of other parts. I went with a 6.5 hp tecumseh and comet 30 series because I had them already.
One + for industrial/TC is ease of maintenance since everything is exposed, and an industrial engine has less wiring/electrics and a far less finicky carburetor. Overall, its just a much simpler setup.
All that said though, if you have the 90 and it will fit, go with it or consider a 110cc for a little more power to make it last a bit longer. I also stretched and raised the chassis for future growth.
This past season i revised the seating and steering as she's going on 10. I may get 2 more years out of it before she has to move into a 150 size buggy. Think along those lines in terms of how much you put into it and how you set it up.
Link to the build thread if you're interested.
http://buggymasters.com/forum/showth...ighlight=shark