The standard main jet is a 170 – but that is probably not your problem!!!!
The main jet only affects mixtrute stength above 3/4 throttle, and at 70mph, you may not be running on the main jet. I suggest you raise the needle first.
Also, check the timing, and make sure the advance/retard is working. Time the engine with a strobe to about 30 degrees advance at 3000 rev/min.
Does the machine seem “overheated” when it seizes?
However, check the oil delivery first, as that may be where your problem lies.
Let me know how you get on.