Approx cost and choice for i /O
 

Suggested fresh water cooled I/0 to drop into 23' Sceptre........including outdrive etc complete.

Figure want at least 40 mph top end.

The 40 mph top end will cost you up to an extra 4G over a 37-38 mph top end. The Tsunami/Sceptre equipped with the OEM Mercruiser 5.7L 260 hp turning a Alpha drive could achieve 37-38 mph. To get to 40 mph, I figure you need 300-320 hp, assuming the Alpha I/Alpha Gen II drive.
You might achieve 40 mph with 260-270 hp and a Bravo III, or Volvo 290DP drive, because of the increased efficiency of the counter rotating propellers.

A quick look turned up this:
Sterndrives.com (not a recommendation)
5.0L 260 hp with Bravo III complete (no shipping or installation) $11,525
350 Mag MPI (300 hp) with Bravo III complete $14,331
MX 6.2 MPI Horizon (320 hp) with Bravo III complete $17,056

Figure $500-750 for shipping, another $1500 for installation, $1800 for the FW cooling package, and throw in $2000 for incidentals like gauges, monitoring systems, and other unexpected stuff. And this assumes your boat is ready for install.

Way back when...the cost of my stroker 383 mpi with bravo and 350 hp. Not FWC was approx. 14k. Top speed just over 50. Fuel used at cruise...8-9 gph and that was pushing 25-27 mph.

Frank - thanks for all the information you put into your reply - appreciated.

Too bad you can't see how fast I'm running........away from this idea........I may even pass by Usain Bolt !!

My Yanmar 6LP 250 has a max rpm of 3800. I cruise at 3050ish....32.5 mph to be most efficient. With the bimini top up and plenty of drag I have seen 42 mph. I bet if I tried hard I could get 45 mph. I get 3.8 mpg too. 76 gallons of diesel. Bravo 3 drive with duo. The guys down in mastry told me the replacement engine is well over 40 now for that engine. Uhhhhhhhhh

Were you to choose to put a bracket and outboard on your boat, you're into even more money.

$5K to rebuild the transom. (Only $1800 if you do it yourself)
$3K for a flotation bracket. (Maybe only $1600-1800 used).
$20K for new 25" or 30" 2018 Mercury Verado 300 hp (Dealer cost is around $18.5K. A NOS left-over 2015 300XL is around $17K, and low hours used 2011-2013 300XL can be purchased for around $11K)

Then you've got your installation to do...