my wholesalers only carry a limited number of Kawasaki engines so it is always a case of finding one that can do the job
Usually it is a smaller engine as nearly all mowers are grossly overpowered
The Z- Master I am currently rewiring had a 691 which was replaced with a 651
The only difference the owner noticed was it uses less fuel
As for electricals they will interchange from one engine to the other as like B & S they all have common mounting points
I can not find a spec for DS08, just A B & C so either it was a "secret " speck with IP belonging to the customer
All you need to know is the ø & length of the PTO , weather it has a key way and is drilled & tapped and the height from the deck to the crank centre
Horizontal shaft engines get used on a lot different types of equipment so the crankshaft PTO ends can be different
FD750D - DS08 has a shaft listed as 49119-2226. which subs to 99916-0003
FD851D - NS00 - s has a shaft listed as 49119-0009
But they both use the same pistons so I will make the assumption that the crank will have a bigger throw .
This link has instructions on how to measure engines for replacement
small engine supplies