The 4 valve norton back in the 60's/70's was built by Sid Mullarney, with guidance from Bill Lacey, who made certain Cosworth parts available for examination in the design stage. Drawings were done by Tony Monk, but the engine was eventually completed and raced with great success by Bob Newby. The Mullarney original was a 350, but subsequently a 500 engine was built and was permitted under the then existing rules.
Some interest has been expressed in building replicas of the Mullarney 500, but all representations have been turned down, citing the rule that if only one machine of its type was made, then only that machine may be used. While this rule is still enforced in Classic racing, the Isle of Man run under their own rules, and machines such as the MV3, Paton and Molnar4v are apparently welcomed.
While Andy Molnar's 4 valver has an incredibly good external likeness to an original 2v Manx, the Mullarney engine top end looked very different.