It should be on the crankcase, just behind the starter. Here is Scott Pickle's 3 main 1940 Riviera #66577. Send us your numbers and we'll post a list.
'All that glitters' department. I quite stupidly bought a car with an engine which showed a Bantam serial number, and did not appear to have been fettled, but which in fact turned out to be an Austin Engine (Austin numbers usually starts with a letter, like "L" or "M" and the crank case has "batwings" and different motor mounts). Seller claimed it was the engine in the car, but the Bantam transmission doesn't work with an Austin engine, and this 'original' "matching number" car mysteriously came with a Bantam transmission. I had clearly bought an assembly of unmatched parts. If you are buying or selling an engine or major parts for it, you should know what the subject of the transaction is. I offer some detailed comparisons here so you can do just that.
Ask around before you do anything. Join the clubs. Ask the many long time members their advice and who is reputable and who is not. There are "experts" out there who are ruining these increasingly hard to find engines by building them too tight, applying hot rodding techniques, using the wrong parts or just not assembling the engines professionally. If you really feel you need more power, install a non-Bantam engine rather than try to hot rod it. Better that than ruining an original engine.
That being said....
There are generally three basic types of engines to consider in Austins and Bantams. I add a fourth section which I hope we can get some comments on. Purists cringe no doubt, but, the fact is there are a lot of modified cars out there, and there is a category for them inthe Club meets.
All Three were derived from of the original 750cc English Austin engine which was licenced from Austin in 1929. The American versions were a "mirror image" of the English parent design, so, some of the English parts, sometimes with a little fiddling, can be used in American (Austin) engines. Pistons are an example, but anything threaded will be SAE on American cars.
When Roy Evans launched the American Bantam company from the ashes of the American Austin company, the Bantam engines were enough different that a royalty was no longer paid to the Austin Company, so, the Bantam, really is an all American car with both body and engine designed in the US, albeit, certainly owing a great deal to Austin. The differences were in the crankcase shape and the main bearings which went from roller/ball to conventional steel backed babbited bearings.
Unfortunately the three main didn't make it's appearance until the 1940 model year (10-4-39), which saw only 800 cars produced (maybe not all three mains either) and there were only 138 cars titled as 1941's (but really cobbled together '40's) released from the factory. Moreover it is unknown whether all of these cars which were assembled from leftover parts all had three main engines.