Engine issue

Bought my 2010 on 18 November. Took it to the only dealership I have ever trusted because it had an engine shake like you get when coil/plugs go bad. I replaced the plugs and coils myself.

When cold it runs fine but after running for 30 to 40 minutes the misfire engine shake comes back with P0301 code.

The dealership which has always been more than honest with my prior Prius is saying after 2 days of tests I need a new engine because of cracked block and I need a new engine. Coolant levels were very low according to them.

There is a hybrid shop about an hour away I called to get a price on dropping a "new" engine in and they are getting me a price but also believe that the issue may be much cheaper than an engine swap.

I have bill of sale when I purchased the car saying they changed the head gasket a month ago. Also dealership says they ran pressure test and cylinders all passed.

I got the money to drop a new engine in though wife will never let me hear the end of this so question is.....

Drop a new engine in and drive with confidence that it is a new engine

Or

Have the independent hybrid garage look over it and if he agrees with Toyota dealership drop a used low mileage engine in it?

Car currently has 197,000 miles and is spotless other than what ever is going on with the engine