From memory the RT40 has a pretty short engine bay, so methinks a 1G would be a PITA to fit. 3S is a nice idea but pretty expensive to convert to RWD. 2T-G is old and parts are getting harder to get. I'd vote 4A-GZE, it's a nice compact engine with decent power, and makes it easy to upgrade to a 4A-GTE in future.
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