1200 engines are about 3/4 inch narrower than 1300+ engines, I think fan housings are the same fitting as long as you get the earlier, pre dog house style but cylinder head tins are slightly different. You may get away with a few mods with a pair of tin snips but tbh it's a long time since I worked on a 1200.
Somewhere like Megabug should have a stock of second hand pieces in stock.
Just in case you're not aware when it comes to buggy exhaust systems you're pretty much limited to dual cannon types as merged typeset ups will only fit the larger, wider engines.