replacement engine

sorry for the delayon my responce. was in st louis fri-sun.
used machine this am, did fine in snow that was not wet. but at the bottom of drive it bogged down, but engine seemed fine ,belt?the shop that looked at it said it might be the valves,and he was reluctant to pull it apart for ability
to obtain parts (1998 engine) he started it at shop and it would not stay running at low throttle( said thats why he thought valves) after use today
went to reduce rpms by using throttle it barely went down in rpms, any thoughts?

says exhaust valve leaking,loss of compression on recpt from shop. would this be fixable at this age of engine 1998? able to get parts?

was told that they are hard to get parts for,and it would backfire only when
i pushed throttle down fast.

where are the serial #S on this motor? the book says its on the sheetmetal
on the front, but it is not there , hmmmm