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This topic came up in the discussion with the 63D 50 hp Yamaha gearcase. The earlier 50 version was the 6H5 model. They were very popular on VooDoo’s as long shaft and on delta’s as short shafts. Hydro Tec did provide specification sheets to APBA when the motor raced in Formula 50 class in the 1990’s.
The motor is very good in stock form. Weakness is the crankshaft. It can be corrected by being welded. Weakness is common with all modular style cranks. Comparing the APBA sheet to the production motor finds there is minimal difference. Not much can be done if you want to stay stock.

In SLT (super light tunnel) I only know of two builders of mod 50 Yamahas the had success. The fellow in Florida was the better of the two.

When asked about mods, this is what he suggested:
“ on the cdi to get rid of the rev-limiter snip the gray wire coming out of the cdi.
....set the timing to 22*.....if you have an off centerfire cylinder head try and put the centerfire one on.......if you want to get into porting widen and raise the exhaust port 1mm and blend the other ports. ...... 1" or 25mm off the exhaust tube......try and find a set of 70 carbs or a set of the newer style 90 carbs as they will fit with a few modifications.”

Yet another example of minor exhaust port change and carburetor change on a Yamaha triple that really changes the performance of the motor.
He didn’t mention shaving the cylinder head...I think he does that as well. Likely 160 psi would be a good place to start and it doesn’t take much to jump the compression.

This thread adds more information.
https://www.rib.net/forum/f36/yamaha-2- ... 77960.html

One interesting result is these mod motors turn approximately 7600 rpm with good all round performance