As you are in the States you will find that 1:29 is a common scale but really for this purpose you decide what 'prototype' you wish to follow.. divide track gauge into that and maybe you have your scale.... really not terribly important to those other than rivet counters who wouldn't buy that loco anyway.
American Narrow gauge is generally 3ft though I know of two 2ft lines while 'standard' is 4ft8.5 in.
Never met or seen a 'single wheeler' which is a french way of describing a loco ... Brit/US normally would say 0-2-0 as we count wheels on each side/
Honegar's 'Pacific 231' is music for a French film about a Pacific loco which we would call 4-6-2
I saw the film in my student days 1952 at film society screening . music occasionally on radio.
I have adjusted plastic models with hacksaw and hot iron .....Try Roundhouse in Uk for an attractive replacement smoke box door/cover. They have a good website to browse. They make live steam for 32mm and 45mm gauges but also sell parts and kits for electric as well as live steam.
'The Train Department' is a US supplier who I have dealt with happily ... as I have Roundhouse. Both accept credit cards which make payment easier for me not in the States
There are also plastic toy locos which could provide a smokebox door for you ... your estimate if suitable