>>1901051Reduce driver diameter and cylinder bore
Add larger grate and bigger diameter boiler
Add economiser and another feed water heater utilising steam from the front cylinders.
Condense exhaust of rear cylinders and put back into the water tank.
Add re-superheater in steam circuit between HP and LP cylinders
Sort out blastpipe/chimney proportions
Though the real answer is probably this
>>1902405. The Triplex didn't have a big enough boiler for all those cylinders and a Mallet-Garrat has few restrains on those dimensions.