Replacing a connecting rod on snowblower engine

check for loose shaft bearing allowing our of alignment
check for something jammed in the end ( toy, rake metal rod etc

could be any number of things. A bad valve (seized). the only way to really tell is to tear it down and go from there.

ARE YOU REPLACING WHOLE THING OR JUST THE ROPE?
if replacing whole thing just unbolt the 4 nuts

The issue here is likely related to the "linkage". Linkage is the parts (often a cable or metal rods) that connect the shifter to the actual part(s) inside the snowblower that need to physically move to change speed and direction of the machine.

You'll need to carefully inspect the entire linkage to locate missing or worn cotter pins, bolts, etc. You may need to open the underside on the snowblower to inspect and access the entire linkage. These are typical parts that are used to keep the linkage connected to the shift handle and internal wheel assembly. The linkage isn't usually complex - often a pin or threaded portion that passes thru a hole in an assembly - that is held in place with a cotter pin, bolt, etc. Simply move the shifter so that the linkage can be secured properly and you should be "good to go".

Good luck!

Small hole or bolt that sticks out.

For the 12 Pointed Head Bearing Cap Sub-Assembly Bolts
Start from center bolts and tighten in a clockwise rotation working out
Step 1 all bolts 22 Nm or 16 ft lb
Step 2 all bolts 44 Nm or 32 ft lb
Step 3 all bolts 45 degrees additional clockwise rotation using an angle indicator.
Step 4 all bolts 45 degrees additional clockwise rotation using an angle indicator.

Hexagon Head Bearing Cap Sub-Assembly Bolts
1ZZ-FE Engine 18.5 Nm (14 ft.lb)
2ZZ-GE Engine 18 Nm (13 ft.lb)

Connecting rod:
1. Apply a light coat of engine oil on the threads and under the heads of theconnecting rod cap bolts.
2. Install and alternately tighten the 2 connecting rod cap bolts in several passes. Torque: 1ZZ-FE: 20 Nm (204 kgf-cm, 15 ft. lbs.) 2ZZ-GE: 30 Nm (306 kgf-cm, 22 ft. lbs.) If any of the connecting rod cap bolts does not meet the torque specification, replace the connecting rod cap bolts.
3. Retighten the cap bolts by 90 more
4. Check that the crankshaft turns smoothly.

remove the cylinder head you will see if a valve seat fell out or a connecting rod broke or even a bad gasket

It might be u threw a rod sorry hope it helps ya

Knocking sounds in the lower engine area usually are from loose/damaged connecting rod bearings. Repairs are centered around resurfacing or replacing the crankshaft and the associated bearings. (many penny!)