Bearing stock can be found at industrial supplier such as Motion Industries
and is available in range of sizes with center hole.
The sequence of work I leave to you to determine. It is a job calling for
a high level of skill and some tooling. I asked a very expert machinist friend
to do the honors on a lower cover bearing on my Mercury K2. Making the old
new again calls for dedication but when you are done you know it is right.
From what I can determine from parts list replacement crankcase and cover were
sold as units...probably because the bearings had to line up if crankshaft
is to turn easily and be supported by bearings. If you have the shop and
skills a very doable repair.
You can source hobby metal needs on line but be sure you get a brass alloy
suitable for making a bearing.
Joe Brincat President of Great Lakes Chapter does machinist work and likely
could give you a quote or fill you in on how to go about it. Joe's
business card states reasonable prices and 40 years experience.
Boring out enough material to convert to cartridge needle bearings would
remove more material than I would feel comfortable. Far easier just use a KD3 or WD3 crankcase and avoid machining if needle bearings desired.