I had a 1974 Elsinore 250 this is not it, but pictures of what we are talking about:
This guy has an excellent restoration page on ths year bike:
CR250M (1974) | 32:1
Here are the highlights:
There were a piston port engine that I recall, NO reed valves, no low end torque:
Here you can see where he has added reed valves:
I loved this bike, for its day it was awesome, but it was extraordinarily hard on gas, would not idle and low low rpm torque.
My 2000 KTM 125 makes more power and had better low rpm torque and uses 1/2 of the fuel.
Besides, the gearbox was very close ratio, shift forks broke several times and the dogs wore easily.
Sorry to say, I would not recommend it.