There are several things to consider if your riding lawn mower is losing power.
The first is age. If your riding lawn mower is over 10 years old and is moderately used, it could begin to lose some power.
Second is the number of hours. If your riding lawn mower has over 500 hours, it could also begin to lose power.
The two scenarios above are focusing on the mowers engine wearing normally and becoming less efficient. This is part of an engine wearing out and there may not be anything wrong with it.
If neither above is true, then you have an issue that needs some troubleshooting. The first thing to do is to check for ample spark at each cylinder. If you have good spark, it’s time to move onto a compression test. Test each cylinder and you want to see a minimum of 100psi. Preferably you want to see 150psi. On twin cylinder engines, if you see a big difference between the two cylinders, you have a problem.
If you have low cylinder pressure, the second test to do is a leakdown test to determine where the air is leaking out of the engine. It can be the valves, the head gasket or the piston rings. If the leakdown results come back good, you can suspect that the intake and exhaust valves are out of adjustment. The engine could also have overheated and caused a valve guide to move towards the rocker arm and now the arm will not move the full travel and ends up bending push rods. In this case, change the cylinder head.
If air is escaping from the piston rings, then you may have a broken piston ring or they are worn out.
A lack of fuel or air can also cause your riding lawn mower to lose engine power. Check for clogged air and fuel filters or a fuel pump that is going bad. Your carburetor could also be dirty and require cleaning.
These are the main things that cause an engine to lose power and by systematically checking each one, you will be pointed in the right direction to determine the cause of your riding lawn mower losing power.