Can't comment on the Perkins vs Lehman but the 4-108 is very similar in size to the Beta 50 (BV2303). I replaced a 4-108 with a Beta 50. It is a little narrower and slightly longer than the 4-108. It is also lighter.
IMHO the Beta 50 is a far superior choice than the 4-108 which is why I chose to replace rather than rebuild
. The Beta is more powerful. The 4-108 is not really a 50 hp engine
, more like 35 unless you run is at 4000 RPM
which no one does. The 4-108 is an upgraded 4-107 which is a 1940's design. The Beta is a cleaner running engine. Both are very reliable. Installing a used 4-108 that has not had a full rebuild is just too risky and a fully rebuilt one is would be no bargain compared to a new engine.
When I was deciding what to do I wrote down everything that would like to change on the Perkins and visited the Beta factory. Things like re-positioned oil
filters, second alternator
. They were able to do everything on my list.
That was sixteen years ago, I still have the same boat
and I am still happy with the choice.