The old five valve 660 motor bolts in but not sure about the current motor.

Other things to consider would be the oil tank and airbox and there is not alot of room on the TTR for the electricts etc.

I am pretty sure you would need an engineer cert in most states in Aus for this sort of mod.

Also what model TT600R are you looking at, 98 was the year of change (in Aus anyway).

It would be nice to have that 660 grunt in the TTR frame.
Anyway I have my TT600R on the bench at the moment so will have a look at engine mounting etc and compare it to the 660r if you like.