Wondering what effect taking out maybe 300 to 400 pounds from way down low and adding 50 or so pounds way back and up high might have on the sailing characteristics of your boat? Maybe making it a bit tender? Seems like a big change in the overall design function? If you can find someone that has made the same change it would be interesting.
The void where the engine was, probably will not remain empty. He'll load it with gear which will probably offset the missing weight somewhat.
Now that I'm home, the gear looks solid, and well-made. I really encourage you to look for a 5 - 8 hp TWO-STROKE outboard. It'll be very light and give you a better push against an adverse current. If you're going for a 4-stroke engine, then yeah, 4hp is probably your weight limit.