Need to replace the rotostop .

Check on Honda engines.com

try this link from europe

well thats a surprise to me its in english
and download actually works

http://www.honda.co.jp/ownersmanual/HondaMotorEurope/power/lawnmower/hrx/hrx537/english/32VH7Y14.html?c=lawnmower

sorry to tell you this but that will never go always . It has a bent crankshaft usually right by the blade. It will need to be rebuilt with a new crankshaft. Oh, use can use it like it is, but your hands will fall asleep with vibration.

Clean pulleys, replace belt.

I think your mower has a blade clutch. If so, does it surge when the blade is engaged or only when disengaged? Either way it usually means there is some sort of debris in the carburetor. If it is surging with the blade disengaged, that is usually an indication of excessive wear in the carb and I would recommend to replace the carb. Otherwise, it can be cleaned.
Good luck!

I'm guessing the blades operate from an electric clutch and you have a short related to the clutch. Check your switch, wiring and clutch for bare wires shorting to ground.

It would depend upon WHERE in the world you are located but you should be able to contact your nearest Honda Mower dealer or do an internet search for
>> "hrx 537 honda parts" << and it will return quite a few places both on-line and in-person place where you may purchase them

Sounds like you lost the drive key out of the crankshaft, it's usually half-moon shaped and keeps the blade hub and the crankshaft turning together, the bolt just holds it all on the end of the crankshaft.
Hope this helps!