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Stock SST 60
There are APBA spec sheets for these motors to follow.
The motor will have approximately 180 lbs compression and if you wanted to race in T850 you need to run a larger cc head to reduce the compression to 160 lbs.
You can install SST 60 sleeves in the production (sand cast 1986-7 block) That block flows better than the SST 60 foam cast block.

Stock 56 fishing motor bridge port sleeves
Straight forward stock rebuild . Original motor had approximately 120-125 compression and shaving the cylinder head to the SST 60 head spec increases the performance by increasing the compression to approximately 160 lbs.

56 oval port ...the sleeper in the group
OMC engineers de- tuned this motor to get it down to 70 hp at the prop.
Stock the motor has approximately 150 compression.
Very large exhaust ports
But, compared to the 56 Bridgeport
- smaller carburetors
- smaller intake ports
They all reduce the air/mixture flow.
Crank shaft is fine spline and wouldn’t work on a nitro gearcase
Sleeves have been known to twist in the block so have the block checked.
Changes
-4 spline crank
- big bore bridge port carbs
- port the intake ports to SST 60 spec
-surface plane the block and head
- try the earlier 56 bridge port ignition.
- plastic reeds
- larger main jet..76D
Piston rings....just my suggestion..I like the square shape rather than the tapered shape rings on the oval port,

I was asked for suggestions in modifying 56 OMC powerheads.
I have never modified a 56 OMC.
Those that have normally don’t want to tell their secrets.
Other major engine builders may sell complete powerheads at a price most couldn’t afford or they only sold these special powerheads to a selected few.
The intake port enlargement suggestion on the oval port was a suggestion based at just looking and comparing the different blocks. I know some guys have tried it and it works.
One of the best suggestions I ever got was direct from Yamaha Japan from one of their tech engineers. I had contacted them because new in the box 70CES didn’t perform to these 100 mph record speeds shown in the record books....on a prop dyno test my motor peaked a 70 hp @ 6400 rpm. It would be impossible to that motor to push a tunnel over 100 mph. For an actual tech to directly respond was a shock! We communicated back and forth and he confirmed my 70CES was right on spec. I asked if he had any information and was there a 90CES in the works at Yamaha. He said no, but he had played with one. He faxed me a hand drawing of the ports and said that produced 120 hp at the crank with a 165 compression head on pump gas. That was in 1997. A few years later I had a powerhead built to that spec. I used a hydro tec billet head to get 165 compression. The powerhead dynod 119.5 hp at the crank at 6500 rpm .
Years later people like Jason Johnson started modifying 90 Yamahas for drag racing. The modifications were extreme with first builds and over the last few years have become less extreme.
Lesson here
The experts who manufacture the motor likely are more knowledgeable than you will be. Engine mods is a trial and error process for most people and can be very costly.

Great stuff Don
Cheers