I just did the 40 amp upgrade to my '94 a couple of months ago. I am probably the last one to do it, cause I still have John O's handy dandy roaming install kit. Wasn't that hard with all the info available on the process. Well worth it.
I think the main thing here is that if you have a 28 amp alternator ST, 91-95, sooner or later the alternator is going to go south. It's a bad design. Mine started leaking oil like a sieve at 24,000 miles. You don't have a lot of options when it does. When mine went I was starting to think that I was suddenly stuck with an 1100cc boat anchor, cause if you don't have an electicity maker, the bike is useless.
The main parts, other than a 40 amp alternator which you can get through Walmart, are the shaft and bearing, both of which are currently still available. 13411-MAJ-G20 and 91004-MT3-003.
It took me three months to find a 40 amp alternator base plate at a salvage yard. It cost me $260 when I found it. That doesn't include the hours upon hours I spent on the net, sending out hundreds of inquires to every salvage yard I could find, then wading through those hundreds of negative replies I got back, getting more and more depressed every day. I only found one stateside. Found a second in Europe that I bought as a "just in case the first one didn't pan out".
So the bottom line is, if you have a 91-95 with a 28 amp alternator unit, it's only a matter of time before you're going to need to replace it. The plate is no longer available from Honda. Originally they were like $55, but there are no new ones. Used ones are like gold as there are only so many 96+ ST's in salvage yards and the salvage guys are leary about selling just the plate as it's pretty labor intensive to remove, so they'll want to sell you the whole shebang. And often they don't know what they are actually have or are doing as I got sent a 28 amp unit that the guy swore was off a 2002 bike. I'm still trying to get the money back from that one.
In my hunt, I found a second plate, in Europe. I only found two in over three months of searching. I sent it to Larry Fine who is attempting to have some plates manufactured from the one I found in Europe. They aren't available in Europe or anywhere else for that matter either, and there are grundles of 91-95's over there. If I had a 28 amp unit, I'd be getting with Larry and get me a plate made now and would also be getting a shaft and bearing while they are available. Last I heard Larry thought he could get them made for about $300 a piece for ten or more.
The shafts are about $50, bearing is $17. So for $370 you would be able to ensure that your bike wouldn't turn into...a boat anchor when the 28 amp alternator goes south, which it will. Just ask the two or three hundred guys who have already done it. It's not that much money. You spend that much on tires every couple of years. Something to think about.