[Leo.G (Leo)]

If the scores are big enough to feel you will have compression issues, even new rings and piston won't fix that. If the cylinder isn't too badly scored with fingernail catching score marks a quick hone and a new set of rings could improve compression.
The solid carbon you can remove with a scraper or soft wire brush. If it's not the gasket mating surface you don't need to be as particular as to how gentle you are and a wire brush on a drill will do it as long as you don't gouge it into the alloy.

I'm sure you'd know the old trick of a little oil down the plug hole, if it boosts compression the rings are shot.
If the score marks aren't too bad even just new rings may give your engine enough life to mow and maintain the yard without bothering to hone the bore.
As mentioned lower, let me know which engine and I can see if I can hunt a standard ring set up, if they are laying around as old stock I may be able to get them cheap or free.

EDIT: Top left side of head (gasket surface) just below head mount hole (second image), that isn't a crack in the cylinder head is it?
It could just be a hair or something but if it's a visible crack as it appears the head is shot and you'll never get compression.


To remove the solid oil/carbon possibly jamming the exhaust valve open turn the crank until the exhaust valve is opened and use a compressor to blow the obstruction out. You can also spray carby cleaner in which will help break the carbon up, I use brake cleaner or carby cleaner to do that, whatever is at hand. Just flush the crank out with old oil before putting fresh oil in to use the mower.
Oh, WD40 will also break it up slowly.
If it's carbon build up on the valve seat your are best off removing the valve. Few people lap valves these days, I'm past it but you could use a soft wire brush and carefully clean the seat, or grab some fine to medium valve grinding paste (very expensive now) and a lapping tool and clean it up that way.
You could have some laying around, it's also used to frost glass (or used to be used for that).


Always tilt the mower to the exhaust side (carburettor up) to prevent this issue with 4 stroke mowers.


A friend who owns a local mower shop has 34 acres with so many old mowers for parts. If you were in Sydney the chances are I'd be able to get you a good replacement engine basically free. Freight to Victoria would be a killer.


If you give me the engine model I'll see if I can hunt up a service manual online?
They are mostly available free for Briggs and Victa engines.


Good luck!