i have wonderful news for you. you don't need the oil cooler (heat exchanger). and this according to the perkins 4-108 shop manual.
the 4-108, as you probably know, was not specifically built for marine service
; most of them went into farm tractors like your massey ferguson and other industrial/agricultural applications, where heavy loads are more common and more continuous - like pulling a five shoe plow through 640 acres of canadian loam.
perkins workshop manual, 4.108m series, page m-8;
" where engines are operated under light loads or at low speeds, which can result in sludging of the lubricating oil, the oil cooler can be removed or bypassed on 4.108 low line engines providing the raw water
temperature does not exceed 100 degrees F and that the engine speed does not exceed 3000 rpm
. later 4.108m lowline engines are not fitted with lubricating oil coolers, as tests have shown they are not needed. "
by the way, raw water temperature refers to the sea water around the boat, so unless you're sailing through a volcanic eruption with ash falling all around you, or maybe through a hot spring, it's probably ok to bypass the oil cooler...