There have been a number of articles written both in
publications and on line for very good substitutes
in lieu of the highly priced factory parts.
I built my first interface in 1992 from a article
circulating the BBSs. It uses a MAX-232 chip which
for years was at every radio shack. I suggest doing
a search on "Max232" and seeing what you find. I do
know a Russian digital op has a diagram and parts
list on his website for almost an indentical version
of mine. I saw it several months ago while looking
for something else. I failed to bookmark it.
I loaned my original printout of the '92 interface to
someone and it did not come home.
Here is the URL for a diagram and information in
Do a search with Copernic or similar high grade search
engine and you find tons of stuff on the MAX232. I
saw interfaces for Icom and Yeasu before finding this
one while ago.
Also remember you will need to add the two memory ICs
inside the rig. I'll try to find the ID on mine.
The 440/450/et al work very nicely with a computer. I
used a 9v battery for the power supply and it would
last almos two months. I built mine on perf board and
even with a 2 kw amp a foot away I had no problems at all.
Hope this helps.