Pls elaborate what is under powered. You are suggesting him to swap with vitz engine whereas he is restoring his car to original condition.
How the engine will be mated with this back wheel drive car. What differential is required to match this cars body, wiring, brakes upgrade, etc etc etc.
Its not an easy task. Talking about swapping things seems easy but practically its hell of a difference.
Alteration is always done in such cars where parts compatibility and availability is easy........