The 300E and 190E 2.6 may feel the same while driving, but that M103 2.6 was shoehorned in to the engine compartment of the W201.

I fell for the same trick. I had a W124 and 190e both with M103 engines. I bought kits of fuel,air,and oil filters from a wholesaler for the M103 engine expecting the airfilters to be the same.

The smaller profiled aircleaner element was harder to get and more expensive than the larger ones for the W124.

I would change the airfilter housing back to the correct size for this reason alone. You should be able to find one at a wreckers for a very small amount.

The transmissions for the 2.6 and 300 are different. Don't get caught out with this.