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Hi, my name is Kerry
Yanmar offers three different transmissions with this engine, the KM4A is an angle drive with a cone clutch and the KBW20 and KBW21 are both straight drives with a multidisc clutch pack. The KBW21 is the more durable of the two.
Why do you think it's the wrong transmission for your application?
The KBW20 and KBW21 are identical except that the KBW21 has it's gears specially treated to be harder and more durable. The KBW21 is used on more powerful engines than the KBW20. Aside from the temper on the gears these two transmissions are virtually identical and are available in the same gear ratios.
The KM4A is a transmission with a cone clutch which may be more to your liking but it would mean that the engine orientation would need to change because of the angle on the output flange.
I seems to me that the transmission you have now should be able to shift fwd/rev/fwd/rev just like any other marine gear.