1. get yourself a service pack ie plugs and air, oil, fuel filters just to keep to hand.
2. Take the spark plugs out, inspect them to make sure they aren't falling apart. Check the colour of the deposits on the terminal at the end of the spark plug - sandy brown and they're fine, black would mean burning too much fuel. Get a set of feeler gauges and check the gap across the terminal (find correct gap from manual) carefully adjust if needed. Wire brush the terminal up to clear off deposits. Put back in engine and check running of machine. Still no better then change them for new.
3. Air filter. Check it, has it gone black and clogged up. If it has then change it.
3. Fuel starvation is a posibility. Bit hard to check this unless the filter is a see through one. If it is remove flush out with clean fuel and/or high pressure air. If it's sealed unit change the filter as a matter of course.
4. Fuel pump. rarely break down but can after years of use when under high demand/heat.
5. Engine timing. If this is out machine will either lack power or back fire, plus the signs will be seen at the spark plugs with the deposits on it.
I don't know anything about this machine but as a qualified mechanic this is the basis for fault finding.