It's a Jawa motor (shape of cyl' head & clutch pressure plate), chassis? Yay, another speedway bike with unknown chassis !
I would just call it a Jawa 500, you can buy a complete machine off the shelf, with GM you buy a motor and shove it (technical term) in the chassis of your choice. I think Weslake stopped building Speedway engines in the 80's.
High pipes, it's a Scrambler