I can only speak for the Truckzilla conversion kit. The .46 engine is really powerful and well worth the investment. Not sure why you think it is a lemon, there certainly were issues with the first few engines since it came out. My advise though is that you need to make sure you are comfortable with dealing/tuning a nitro engine. Not suggesting that you are not at this time. But going from a stock set up to a Truckzilla is quite a big jump if you have not dealt with nitros for a while, that one really needs to have a lot of patience and understanding on how to manage and maintain a nitro engine, much less a .46 engine.
On the other hand, I have heard of a lot of good stories about the .26 Picco. In fact, I did ask in this forum about its performance before I bought my Truckzilla conversion kit.
My advice to you is go with your gut feeling, so that in the end you would not have anyone to blame if the engine you chose did not perform as you expected.
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