Hi Kev,
At any point have you put straight unleaded fuel into it, rather than 2-stroke mix? If not, what ratio are you using for your 2-stroke mix, what oil, and what fuel?
You should see little to no scoring on the piston if it has had the correct mix in it, the oil in the fuel will lubricate the cylinder and prevent wear.
My first thought would have actually been a fuel flow problem, considering you looked at everything else it seems. That fuel flow could have come from the filter, a kinked hose, or a blocked or the wrong jet. My second thought is that you may need to install the service part - washers - into the carby.
You can perform a compression test to see if you're having an issue with compression, but in my experience, even if compression isn't that bad, you can still have issues starting a scored bore/piston Victa 2-Stroke. The only things to resolve that issue would be to have the cylinder honed or re-bored, and an oversized piston put in.
If a Victa 2-stroke is looked after correctly, it will last a lifetime. The problem is that if you go through all of that process and have it fixed up, you'll end up with a lower life of the mower, because while an over-sized piston is a viable option, it can end up causing wear itself, so your experience with that engine may not be that great.
My recommendation, if you have a good body (Alloy, or steel that hasn't rusted) and you're happy with it, try to find a replacement/second-hand engine and swap it out. You need to be mindful of the length of the crankshaft and the blade boss that's installed, but if you get the right one, you should be able to unbolt the old one and bolt on a new one that has no wear and you're good to go. Alternatively, buy a second-hand replacement mower, that could end up cheaper. Just check that it doesn't have any scoring, make sure it runs correctly before you buy it, and even if it has some issues, you have spare parts from your mower
As long as it hasn't had straight fuel through it, you should be able to fix pretty much anything on the mower for a relatively low amount. So before potentially going and spending $450 on a 4-stroke from Bunnings, ask yourself which will last longer, be cheaper to run over its' life, and be easier to service?
pau13z
