What engines are those, Dave?   High output singles, very light looking (discoloration may indicate magnesium cases, barrels and heads).

Whose are they?

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Click on it to open it -- GR engines  (alias Godden Racing)

https://www.facebook.com/media/set/?set=o.190927679332&ref=mf

The bikes are basically European Speedway motorcycles with a front brake and a gearbox added.  They are 500cc, single cylinder bikes - they don't have a rear brake as you just throw it sideways to slow down......the only engine I know of is the Jawa but there may be others.  ( I think the sidecar bikes use a bigger engine).

Looks like it'd be hard on a fellas fellas