he suspects it is the bore or crank that has worn.
“Suspects” worries me. At the best an educated?guess. As AF has suggested, lift the head and inspect the bore for obvious deep scoring or galling. If all appears to be well, there’s not much more to be discovered without serious dismantling and measuring. You don’t mention the make of engine fitted but if it’s a side valve Briggs and Stratton I’d have a look at the valves. A valve job / top end overhaul can be done without special tools but a couple do make life easier.
Take the valve chest cover off and before going any further check the valve gaps. Most BS side valve lumps need 5-7thou on the inlet and 7-11 on the exhaust for early ones and 7-9 on the later. Measured with the piston 1/4” down from Top Dead Centre on the power stroke. If the clearances are tight it’s an immediate pointer to go further and remove the valves. Inspect the valve faces and seats and also for any signs of a loose seat. Particularly a loose seat as they can come adrift when the engine warms up but sneakily drop back into place as it cools.
No point in me going any further until those “suspects” have been checked.
If you don’t feel up to doing this, either find a slightly more enthusiastic engine man or, as AF says, fit a new engine. Loncin happens to be a favourite of mine.
PS. When it cuts out, have you checked that there is still a spark ?? We may all be up the wrong tree!!
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