Engine interchangeable between a MkI and Mk2 Austin 1800?

Not a big problem. However you need to be aware that the MK1 engines used rear mounted SU electric fuel pumps and there is no take off for a mechanical one. The cylinder head for a Mk11 car is not compatible with a Mk1 engine as the latter block has scallops to provide clearance for the larger valves.

The Mk11 gearbox has a higher ratio 2nd gear and I seem to recall a stronger layshaft so would be preferable and should bolt straight on.

The inlet/exhaust manifold and carb from the MK11 would fit the Mk1 (and vice versa) and you would need to change the SU needle to the Mk1 version. It may be preferable to retain the Mk1 manifold and carb if it is available, noting the carb is set at a different angle from the Mk11 so you would need to change over the casting that sets the float chamber horizontal (30 vs 20 degrees)to suit if you want to mix and match.

You should try and retain the Mk1 distributor, I think some Mk11 Oz cars used an electronic distributor and I am unlear as to whether the advance curve is close enough. The UK Mk11 distributors can be used on Mk1 cars using the Mk1 static setting without too much loss of performance.

Australian Mk11 cars used stronger modified engine mountings but I don't think it is a problem using them with a Mk1 engine, but I suggest you take advice locally.

There may be other changes that I am unaware of as Mk11 Oz cars had a number of differences from the UK specs, hopefully someone here can pick up any I may have missed or got wrong!

David