Triplecrown put together a completely stock spec engine for member SHart.
It had no problem achieving 50mph, so I would suspect your sprocketing is off.
Are you running out of power or revs? (And don't tell me both.)
Beyond 50mph it takes ever increasing amounts of power to build speed.
The first thing is to get your sprocket ratio right for your tire size.
Now is the time to tune the engine in for perfect operation, leak test and plug chop for perfect jetting.
Not only will this give you the most power and speed, it assures the engine will not blow up.
The first mod I would do on any Blaster is the head rechamber and set up the squish, followed of course by a leak test and plug chop.
After that, a pipe, maybe a hole in the air cleaner lid, and of course a leak test and plug chop to set her up right.
Beyond that, some porting or even an extra basegasket to up your rpm (adjusting your head to match).
This will get you to over 60mph and all the wheelies you can stand.
Did I mention leak test and plug chop?
Steve