While I have never been on a 23 Maycraft, I owned a 25 with a single 225. The boat did well over 40mph with the right trim and cruised in the upper 20's. One of the benefits to a shallow transom deadrise is not only stability but you are able to push a boat with less power (giving great fuel economy). Heck I am now pushing a Judge 27 with a single 225 and it is more than adequate. Being that the 23 Maycraft is a small 23 at 22'6" with a beam of 8' and it sports a tapered deadrise hull, My $10 says that a 175 does well on that boat. A seatrial would confirm.
If you want to be correct for insurance and all that stuff check the HP rating on the Coast Guard Plate attached to the hull.
If you want to get to the fish real quick go with a Merc 250 XB and a 15.25 x 23 pitch prop. The larger prop will net you better gas milage at cruising speeds and the push for when the fish are biting and you need to get there in a hurry.
The torque of the hpdi will help out of the hole. That motor wieghing 100 lbs. lighter than a four stroke won't hurt either. If the beam is 8.5 ft or less and hull weight is around 3500 you should do fine. Do you know the deadrise?
My buddy runs a 23 maycraft and has a 225 smoker. I feel it is well over powered and he is thinking of going to a 150 four stroke that boat will run 50 with that old 225 it also drinks gas badly. I own a 200 HPDI and have it on a 24 W/A and can do 35 with the rght conditions.It only burns 9 gallons an hour at 4200 rpms.