I think most from BMW would fit, the smallest would have been from a 518 M10 or 316 M10 and apart from having a small torque converter the A clutch would have less plates, it would work but might not last too long, you want a simple version with a kickdown cable not the EH electronic type with a 7 or 8 pin plug, there was a version with a two pin electronic plug AND a kickdown cable, that works ok, just ignore the wiring, look for a 4hp22 from say an E28 or E23, should bolt in easily, the bellhousing must be from an M30 engine, an M10 would bolt up but you are back to a smaller converter, I fitted a 4hp22 into my 1972 2002 and drove it for 10 years as a daily driver, worked great, very low rpm at 65-70 mph, I kept the 3 speed shifter, who needs to be able to shift it back to first?