If I recall correctly, the Marlin supplied sportster rad is actually a 316 / 318 spec unit for the M10 series engine:
Nissens Part Number : 60678
Material : Plastic/Copper
Product Height (mm) : 350
Product Width (mm) : 443
Product Thickness (mm) : 33
Replaces O.E Part Number : 1.151.700 1711.1.151.700
(Details nicked from an Aussie eBay item, but as Nissens is a UK company the product number should be ok - but just in case it's an E30 316i, '87-'94, Manual, No A/C)