The Heizer JAP Mk1 was built in 1959 but apart from that there doesn't appear to be much information about this lovely little car to be found. However over on the 500 owners association website, as well as several sections on results, every marque that raced in 500 cc Formula Three (or so I thought !) etc there is a section giving contact details of the committee one of which is Gordon Russell who I believe may be related to Michael as both are prolific motorcycle, sidecar as well 500cc Formula Three racers. Both raced in the Earl Of March Trophy and the Barry Sheene Trophy with Michael sharing the Barry Sheene trophy victory with Michael Rutter. So try
https://500race.org/our-club/whos-who/ and with a bit of luck you could find out what you want to know.