Greetings Cool,
In all the years that I've seen the topic of 164 engine swaps discussed in this forum, I don't recall anyone ever suggesting what strikes me as the stunningly obvious choice for more power with the least "reconstruction";
one of the BMW straight-sixes! If you want to keep the heartaches to a minimum (and if this is your first swap attempt--YOU DO), stay with the same block layout--inline sixpot.
If you choose one of the pre-fuel-injection engines (prior to '74 for BMW, I believe), you won't even have to fool with swapping electronics.
But if there are emissions inpections in your area, remember that any non-stock engine will render the car non-registerable (of course, a portrait of Ben Franklin, slipped into the right hand out behind the inspection station, will solve this dilemma in most parts of our Glorious Republic).

Gary in Arizona--with two 164's.