Hi, donno if you sorted yourself out yet with the ninja but here is my story
I also bought an used MR21 in perfect condition a while ago for my eight t 2 and had the same as you're describing. Off the ground the engine would rev like crazy and immediately....but on the ground engine was starting out abnormally slowly and after a while picked speed and would rev high and move the truggy like hell....

What was the problem? Well after opening it up and after some research I found out the following.
1. the guy (ebay seller) has put inside an on road piston sleeve in a buggy engine.
2. The buggy engine (off road) is a simple or better stated, normal 5port racing engine while the on road has 8ports plus 3 exhaust (or 1 exhaust and 2 guide ports).
3. the on road piston is a bit larger than the off road.
4. consequently the on road sleeve thinner than the off road
5. and last the turbo button heads follow a same pattern...on road larger diameter, off road smaller. The buttons dont fit the different sleeves.
6. edit the piston pins dont fit either between the two.

My point is, that with the right combination you can be easily fooled as the manufacturer burned to the ground and parts are scarce or not available. So people just decide to put the on road stuff on that is easier to find available and sell.

Finally, let me tell you that I payed dearly for all this, to understand the problem first and then solve it; as I also probably have the last piece of sleeve and piston for the ninja MR21 B01 in the world! (still new by the way, unused and oiled in the plastic bag. Ninja box is an ashtray now) Nothing to do with engine tuning clutches or any of the standard solutions to acceleration and power.

Have a look at the piston sleeve you're wearing and please confirm that this is the case or not.

Hope this was helpful and if anybody would like pictures of the two let me know.