I have a VINTAGE Bosch PKS 54 circular saw with small 160mm blade. Bought from a cash converter for only AUD45 then overhauled & lubricated it.
A portable power tool often use the Universal motor with armature + carbon brushes. There are also switches and with contacts etc.
The greatest enemy of such power tools are DUST including Carbon powder + hardened or melted grease and RUST.
With a good dust chute and quality shopvac sucking though a mini-cyclone, your portable power tool will last longer.
I like to pay for trade quality Circular saw and routers because they use good quality hardened bushes and bearings but wood chips can damage them all the same.
By overhauling, I mean cleaning up all wood chips, worn metal , hardened or melted grease, remove all carbon dust from the armature and use a pen shaped "glass fibre brush" to clean all electrical contacts including the armature and switches. Some fine sandpapering to make sure that trigger switches will never jam in the on position. Flatten the carbon brush ends and replace if necessary.
After that, bearings & gears rusts are loosened with Premium grade WD40 and regreased or re oiled.
Blade rusts are sanded off and blade lubricated with DRY-LUPE.
All adjustments are readjusted so that 0 or zero angle means exactly zero.
The best tool is the pen shaped "glass fibre brush". I have been able to resurrect a dead vacuum cleaner by just cleaning the armature with it. It give a "brand new like bright shinning armatures free of carbon dusts that often shorted the armature and slow down and stop the motor.