idk i was just driving and i think i misshifted and i started hearing my engine rocking and making a really loud noise and now it wont even start, so i took it to a mechanic and he said i need a whole new engine
If you want to keep it a k20z3, bring it to a shop and let them do it right. You probably bent/broke something and then ****** the timing as I'm sure other things so that's why it won't start.
If you want to make this into a 'race car' then go for building it yourself if you're capable, but just realize any block/head/pistons/whatever needs a machine shop to grind them flat/correctly or hone whatever needs to be honed. They also have years of experience and know what to look for. Also there's a LOT more than just putting on a k24 block. A lot of stuff needs to be changed and IMO the k20z3 block is the best because of the oil cooler (which you need a new one of too - that's $260 min alone)
I just went through this with my car, a rod bearing went, so I took the motor out, found the problem, decided it was above my skill level and brought it to a machine shop. Now all is well with my k20z3
Either way you go, be prepared to spend upwards of $5k or more to get it done right!
You will have to pull the motor out to see what is wrong with it. You most likely will be able to rebuild your current block. I personally would go that route rather than buying a used block that could be bad too.