My toyota townace 3c engine has a very hard start after overheating. I have replaced even the piston rings but no change.

Easier to fix the one that you have.

If you change engines , you may also need to change the ECM

https://www.google.com/search?q=7+seater+townace+3CT+engine+to+2.0+diesel

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The piston and the rings are glazed over and stuck into the piston so there is no seal. It's a guess but a good one.

And oil shouldn't be black :>D It should be honey colour to light brown. Changing oil every 6k to 8k miles is the way you make your engine last.

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Depends on how bad it was overheated and weather or not it was overheated because of low coolant level or air or coolant circulation Can be as bad as a cracked cylinder head or engine block, Seized and weakened cylinder rings causing low compression or oil burning, Can also cause scored pistons and liners, or even seize the engine right up so it no longer rotates

check glo-plug operation again
diesels that are hard to start have the following indications:-
engine not cranking fast enough to heat the air for combustion-- fully charge batteries or bigger batteries
worn engines --do a compression test
bad injectors squirting not atomizing---replace injector tips

Check compression--all cylinders. Your rings are worn out, and combustion gasses are overwhelming the PCV system.

best thing to do is to call a toyota dealer(ask for parts)
give them the cars VIN number (the set of numbers on dash behind windshield) and they should be able to tell you the correct ecm

Take your vehicle back to your mechanic, and have him perform a DRY, then a WET compression test. Please provide both readings back to me. In the meantime, ask him what it would cost you to him replace the piston rings. This is your problem. The 2 tests will simply verify me thinking.