You seem a bit short on replies. I built a Sweet William some years ago. The grate/ashpan isn't too difficult.
Basically, on the buckler drawings
Top RH is a disc with 2 square holes, this sits on the outside of the boiler clamped up with 2 screws.
Bottom LHS is a part section of the grate and ashpan, I must admit I'm not sure of the LH part of the section, but the RH shows the far end of the firebox. It's half moon extended vertically to form a plate to retain the fire at the end of the grate. This will burn away very quickly, its a good idea to make this much thicker or easily replaceable. The ashpan is a sheet metal half cylinder. The drawing shows it secured to the 2 end plates with a few bits of angle and some lugs. I had a half annulus flame cut to make the joint instead. The grate rests on a couple of bits of flat - one on the lower part of the plate at the far end of the firebox, one on the first plate.
Bottom middle is a baffle plate to protect the first plate from the fire. I never fitted one.
My engine is store away from my house, but I may get to see in in a few days, if so I'll take a few pics of the firebox.
If this isn't enough, send me a PM and we'll swap phone numbers or Emails.