what engine would be the best for my x-cellerator, and that gives the best performance

OS .18 TZ but you better have strong drive train. I would go with the OS.18 CV cheaper and has plenty of power.

I would say go with the cvr as this is a plenty powerful engine for a st. I have a cvr in my st and it hauls *****. It goes pretty well and you will have to re gear the truck for this engine. It is very reliable and easy to use. It's a good engine and has more power then that st will ever need.