My dad built and finished a simplex about 30 years ago.
this year the boiler failed its steam test so a new boiler is being made
the ashpan not removable is a nightmare, i have the same problem with my titch!,
my dad made a removable ashpan with the use of a dump pin, but be warned the rear axle does need covering as it gets covered in ash and wears the axlebox out rapidly, My dad designed ashpan number three with a cutout to cover the back axle after the second rebuild
simplex is a monster loco, so easy to drive with plenty of power, as part of the rebuild the bore on the cylinders was reduced as we were only running at 90 psi (simplex is designed for 110 or 120 psi i think'
the change was unbelievable so much steam and much faster and a happier driver!!
hope this helps