Toyota? Sorry, there's no way they're getting credit for introducing DOHC engines. Alfa Romeo had been making DOHC engines for cars since the 1920s, and it has produced a family of four-cylinder DOHCs from 1954 to this day, including lots of mainstream-priced performance cars from the late 1960s onward.
Independently (at that time) at FIAT, Aurelio Lampredi's first DOHC belt-driven engine debuted in 1966 in the FIAT 124, and went on to be used widely in everything from the WRC-winning Lancia Delta Integrale to the everyday FIAT Tipo (Mk1).
By the time Toyota started making its AG series in the early 1980s, the technology was well known in Europe with hundreds of thousands of owners. Yamaha and Toyota would certainly have had several Jaguar, FIAT/Lancia and Alfa Romeo (FIAT and Alfa Romeo were separate, independent companies before 1985) engines to choose from and disassemble on their workbenches...
And yes, the Toyota TwinCams, like the Honda VTEC engines, were notorious for needing to be revved to the limit before they'd move. Over here, they were a favourite of the kind of boys who drew donuts on supermaket car-parks on Saturday nights...