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Far more difficult a search than I thought it would be. This model has the potential to be a very competitive motor on a T750 provided you can get it running at its original specifications. Original parts are a problem. Replacement parts are available, but for a race application you would want the original parts.
A few facts about the 700 series ,70 hp , 49 cubic inch , twin carb.
First off it is not even close in performance compared to the 49 OMC triple.
But it is legal in T750 class and the 49 OMC is not.
The T750 class was dominated by the 45 cubic inch OMC twin carb, 2 cylinder.
That motor at SST 45 specs is approximately 67-68 hp.

On a prop dyno a new 70 mercury we tested in 1980 was 64 hp.
The compression on that motor was 145 PSI.
It had to run on premium gas to run at its highest performance.
On a light boat like a delta the short shaft model ran 60 mph ish.

The Later 1984- early 1990’s version was rated at 60 prop shaft hp. That did not match the dyno hp which was in the 55 hp range.

The T750 class motor specs are wide to allow different makes and models to race. Mercury’s 44 cubic inch 4 cylinder wasn’t competitive and we wanted a mercury motor in the class, so we allowed the mercury 49 triple twin carb in.

We have had only one 700 series 70 mercury in the class. Jake Elsey ran a 13 foot Allison for a few races. It has been the only mercury that was close to being competitive with the top running OMC.
That motor was not built to spec and was well over the class minimum weight.

The purpose of this project was to build a spec 700 series 70 mercury and run it on a light critchfield. The motor build stalled because I couldn’t find a powerhead that wasn’t damaged and needed oversized pistons. The 700 series originally ran non power port pistons and those pistons are rare. I wanted a block that didn’t require pistons just new rings. That ended up being the problem.

Lots of these motors around and they’re not costly. Most are long shaft. I have the short midsection and gearcase. Some have compression 120 range and one bad cylinder with lower compression . Parts are very expensive for mercury outboards and machine shop costs as well so avoiding both is a key factor. Sure I want to build a perfect motor, but I am not breaking the bank doing it.

Yesterday I found and purchased a motor....it has 130...130...125 compression. It should only require new rings to get back to the 140 range. But you never know until you take it apart.

Here is a T750 race from 2019......you can see how competitive the 700 series 70 hp mercury is with Jesse Coulter’s top of the class boat. Those two boats are the top two running boats in this race.

https://www.youtube.com/watch?v=rBm5w-UBr7Y&t=197s

Now if you look at the rest of the entries, you will notice a mercury 60 hp twin carb, more OMC’s, a Yamaha and an S750 Tunnel.
The class is a 55-62 mph class. Certainly fast enough to challenge a driver but not as challenging as the T850 class.

Some education about the 700 series 70 mercury being different to the later 60 hp version.
The 3 cylinder twin carb models had different pistons depending on the model year. You will see non power port, power port which have the hole in the side of the pistons, there is high dome and low dome pistons.
Most of the time these motors will stall going into gear if the compression gets down to the 110 psi or less.
It’s a sure sign the motor needs to be refreshed.
The 700 series 70 hp mercury had high dome non power port pistons. The only oversized original piston
was only 15 over. If the cylinder walls were badly scored or the cylinder walls gone barrel shaped you likely needed an aftermarket piston. The 650XS used the same piston and that was a serious problem for racers. The power port pistons didn’t work well in a 650XS. The low dome pistons which non racers used to avoid running race fuel or av gas didn’t work.
As I mentioned before the 700 series 70 hp bone stock was 140 plus compression most were 145 to 150. The 650XS had the same 145-150 compression.
The 70 hp ran best on premium gas. Dealers would sometimes change the timing so the motor would last longer and run on regular gas.
That’s why it’s difficult to find a good 70 hp that you can refresh before too much harm is done to the cylinders.

This father and son basically only work on the in-line mercury...6-4-3 cylinder motors. Some of the videos are painful to watch.
This video is the compression test of the 700 series 70hp that had just had new rings installed on the original standard bore pistons. They compare the pre-refresh compression to the new compression. The motor is close to being back to its original spec .
https://www.youtube.com/watch?v=MOgkyOTBoJQ