Well in my 2005 Tomos streetline catalog it tells many specs. about the bikes. Here are the differecences between the LX and Streetmate:
Tomos LX: 12mm dell'orto 14-12P, Chain (90 rollers), Drum brakes(105 mm) front & back, 2.25-16 size tires front & back, 42.5 Wheelbase, 64.5 length, 30.5 seat height, 119 lbs (dry), 1.06 gallons fuel capacity, 0.16 oil tank capacity, 220lbs carrying capacity
Tomos Streetmate: dell'orto 14mm PHVA, chain( 103 rollers), front disc(203mm) & rear drum (105mm), front tire 2.25-17 & rear tire 3.25-16, 47" wheelbase, 72" length, 31" seat height, 143lbs dry weight, 0.92 gallon fuel capacity, 0.2 oil capacity, 300lbs carrying capacity
In relation to the engines, they are both the alike other than the carb difference. Other wise the streetmate is 24 lbs. heavier than the LX but the streetmate can handle a larger carrying capacity. So it all equals out in some way.