G'day wortluck.
How long is a piece of string? Lots of things in there to wear and produce iron/steel dust. Timing gears, cam and followers, drive side bearing, piston rings and bore. All motors produce a little but if there any bigger than powder I'd look further. If it's big bits it will be the drive side shim breaking up.
I'd clean everything thoroughly, flush the lines (in and out), remove the tank and clean out all the old crap (I use pressure washer). Fill with new oil and check plug in another 100 miles then again at 500.
Cheers
a10gf got in 1st.