Self explanatory really just name the motor, go ahead and use the net, we will never know
the gt750 tread made me think of this engine. instead of reeds on a piston port engine- back around 1981? before cagiva made there own gp500 engines they would use/modify yamaha inline fours and suzuki square fours. the engine in the pic for the most part is a yamaha inline 4 (ow35/47/47r) and instead of reeds they put rotary disc valve intakes on it. what looks like a long intake trac in the middle is a horizontal plate with rotary .The photo is a little blurry, need a hint to confirm,
do I spy with my little eye, something that looks like a fairly large Dell'orto carburetor?
1981 cagiva frame i believe, i do believe it is a yamaha inline of some sort. just with there own design of rotary intake replacing piston port, underneath the plate that the rotarys spin in you can see the pully wheel that the belt turns ,. , dont think it made more power then the yamaha original design. wonder if it ever finished a race? it does look like the outside exhaust exits the rear, the case reworked to accept the rotary intakeBut the 500J had a carb facing forward on the outer cylinder, so that's not right either. Maybe it's a set of PGM crankcases or a Nico Bakker frame or JJ Cobas.