The 250 cc and 350 cc machines, which later materialized as the YDS5(E) and the YR1 and were introduced as normal piston port engines in February 1967, could have looked as follows: (you can "click" the pictures for the large format!)

Yamaha prototype YDS5

The first "mock-up" looks pretty neat, but it is evident that it should be a (too) wide engine. And the details show that the YDS5 cycle was ready for production.