Edit, said TE 3.0... not actually sure which generation of Magna it is at this stage, only that it's a 6G72 24v.

Just checking, but i'm pretty sure the TE 3.0L has metal head gaskets right? Currently diagnosing a Magna with a stuffed head gasket (losing coolant into the block, starts and runs but begins to overheat after a while). If i'm not mistaken and it is a metal gasket, i'd say that's a pretty safe assumption there's damage elsewhere on the engine as well?

On a sidenote, considering the motor may have sustained further damage (particularly to the heads), how much would one expect to pay for a 3.0L V6 motor either new or used? Used is probably not a bad option, i'm sure there's members here who have gone up to a 3.5 or are planning to and would have a running 3.0L motor still. A new crate engine would of course be ideal, but the main thing is to have the car running without breaking the bank. Either will do.

Not my car, just chasing up some info. :)

Used motor your looking at about $500 give or take. Have seen prices as high as $1000 but for a 3L $500 is a good price.

Other questions I cannot help you on sorry :)

I have seen a few where the cooling system has corroded due to lack of coolant and care

Best way to test is to remove the plugs, take off the rad cap and pressurise each cyl, you will get a rise and some bubbles in the cooling system and that will tell you which pot is gone but be prepared for corrosion as this is the most common cause

You are far better off getting a 3.5, you will need the engine pipe, top alt bracket and you can use the injector harness from the car you have, if you go down this path, I can set you up with a 3.5 and all the other parts you will need to bolt straight in, I have several motors here ranging from 90k up to just under 200k old, all in good condition and tested prior to removal with 3 month warranty. Shipping is also no issue either