Hello Medzsor.
I believe this is the GAZ-51 engine, same name as the
GAZ-51 truck it was used in. According to this
site, that engine is a 3485cc 6 cilinder engine that produces 70hp/2800rpm.
Compare your engine to the picture of the engine in this GAZ-51 truck:
As far as I could find out, this engine has been produced for a long time, and was also used in the GAZ-63 (4WD GAZ-51) and GAZ-93 (GAZ-51 based dump-truck) trucks, and in the GAZ-53 and GAZ-52 trucks, the succesors to the GAZ-51, which had a slightly improved engine (75 hp instead of 70hp). From 1964 the GAZ-53 trucks also came with a 4254cc V8-cyl (115hp/3200rpm) engine - these were also used in Volga's.
These GAZ trucks were built over a long period of time, so it was natural to use the same engines in the Volga boats as well:
GAZ-51: 1946-1975
GAZ-63: 1948-1968
GAZ-93: 1948-1975
GAZ-52: 1966 - late 1980's
Also the GAZ-49 (94 hp) and the GAZ-49b (115hp) engines, used in armoured vehicles like the BTR-60, are based on the design of that GAZ-51 engine.
And as far as I could find out, the other type of engine typically found in the Volga's is the ZMZ-53 / ZMZ-66 (115hp/3200rpm, V8-cyl, 4254cc).
This engine was used from 1964 onward in the GAZ-53 trucks, and it's 4WD version, the GAZ-66.
In other words: it should still be possible to find spares for your engine. They produced around 3 million trucks with this family of engine, so I guess that it was considered to be a quite sturdy engine.
Hope this helps!
Maurits