Um, that weight spec'd above is WITHOUT the muffler!

This is the specs on the OS 91fx
SPECS:
Displacement: 14.95cc (0.912 cu in)
RPM range: 2,000 - 16,000 Bore: 27.7mm (1.090")
Stroke: 24.8 mm (0.976")
Compression Ratio: 10.1 : 1
Weight: 550g (19.42oz) 24.3 oz w/muffler

And the ys 63:
SPECS:
Bore: 1.03" (26.2mm)
Stroke: 0.74" (19.0mm)
Practical RPM: 2,000-15,000 rpm
Horsepower: Not given, contact YS
Crankshaft thread size: 8 x 1mm
Weight with muffler: 1.11 lbs (17.11oz)

So that is a difference of 7.19oz. Make your own judgement if you want to add that much weight.