all good machinists have dial gauges - I used to get mine balanced on a regular lathe with a tyre tube going through the connecting rod small end to prevent it flapping about, hook dial gauge to machine and rest its pointer on the axle after first roughing it up with a copper hammer.
I had an old RX115 yamaha, and I'll tell you how i balanced the crank.
individually polish the crank wheels on a crankpolish tool so there is no runout on the dial gauge, next assemble the crankshaft assy and use late setting bearing lock solution (loctite or loxeal yellow bearingloc) Now we have 30 minutes before the solution hardens.
Now mount the crank on V blocks positioned on a true flat surface (machinists also have a true flat). mount the dial gauge on the V block foot and spin the crank by watching the dial gauge, use a copper hammer to align as required. Once done and the bearingloc locks the crankpin - you will experience a smooth running crank.
On a 2 stroke do not use a hammer to drive the crankshaft onto the main bearing, use a puller or press to draw it in. Similarly drive the bearing onto the crankwheel of a 4 stroke by supporting the individual crankwheel.