https://www.talkbass.com/threads/orange-ad200b-replacing-stock-ot-with-mercury-magnetics-oro-200-m.898436/$300-$400 for a 200W OT or PT does not sound expensive to me.
The whole amp retails for $2,000.
You can rarely scratch-build an amp, ALL parts considered, for less than a factory price. Their quantity discount covers their paid labor. (Different when you have a huge junk-pile, but nobody has 200W parts laying around.)
200W tube amps are RARE. I have only seen a couple single-channel amps that size. (And I did gig with three 2X150W tube amps, well over 700W on tap.)
Most tube amps over 80W are real-clean. The total demand for such parts must be tiny. Cost is a lot to do with volume. Factory gets into a 1,000-part routine, costs come down. Arguably big-tube-amp work can be done with compact economical boxes from the Rock/PA supply store.
That schematic can't be complete. It only shows 2 power tubes? No OT impedance spec? No voltages noted?
And be real. What can you do with a 200W amp that you can't do with a 100W amp? Or a 50W micced into a 2,000W PA system?
You could consider buying two sets of "100W" PT and OT from Fender/Marshall replacement parts. But I don't have a clue if the Orange circuit could be transposed to the voltage/impedance zone.