The electronics in the car make in impracticable to put another car engine in as there various computers talk to each other so there would be big issues with a different engine ECU talking to the other computer bits in the car.
I am the sort of person who would have a go at rebuilding the engine but if that is not you I would get a second hand engine TR Spares in Adelaide sell for about $3500 - get it in and sell the car. These engines have a major design problem - overheating, blowing head gaskets and then dropping cylinder liners - your experience is fairly typical - also happens in the MGF and Lotus Elise which has the same engine.
Sorry for the bad news - the cheaper option is fix yourself but even that wouldn't be cheap.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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