Have you had a listen to these to see if it sounds the same? Won't work on my phone for some reason. http://www.ratwell.com/mirror/Muir/Muir.html
Thats all 4 big ends and they've shed their white metal. Drop the oil and it'll looke like a midnight snow globe mix!. Crank regrind and linebore up....
Just buy @MorkC68 engine. Easy and quick fix, or as its winter do a full strip and rebuild, but that will cost you lots of time and money, I therefore refer you to the start of this post
good job i've asked for a book on rebuilding engine for christmas. if it has done the crank why would it of happened as everything was running ok. if it hjas i'll have to replace everything,oil filter cooler etc. might as well get bigger pistons and a couterweited crank just for good measures... will have to look into stripping it and try to find someone to repair the cream crackered bits.
Its often due to oil pump delivery but I also dont know what oil cooling system you have! does it have a wax thermostat and fan assisted cooler or is it just a filter?
i dont use it on a daily basis but its out quite often. the engine case is drilled and tapped for oil filter and cooler using mocal cooler and mocal thermostatic valve. it does not have a fan for the cooler, which is tucked up behind the rear wheel/battery tray area.