The RM motor was still in the buggy when I did the PE400 mod. Not being into motocross until I joined the vintage scene you have to realise that this was a out of date 500 motocross bike ,it was worth more as a registered trail bike and it made a great trail bike.
When I was racing the buggy with T.O.R.C (Trike and odyssey racing club) the maximum size was 500cc, CR 480s were a popular motor to use , I was offered whole Rm500s for $500 then .
Yes Hoony that was Seymour.
Cheers Arno