My Bianchi Strada LX has the "traditional" headset/stem combined where the stem slopes downwards. I would like to consider replacing them so that I can have a horizontal stem. I haven't measured the headset or headtube diameter so I don't quite know what I'm looking for but does anyone have any knowledge or experience replacing a similar generation headset/stem with something more current?

Maybe this question is more appropriate for the mechanics forum.

Are you talking about going threaded to threadless headset? They make those quill adapters, but I have never done it...looks simple enough...perhaps HERETICAL though! ;-)

I'm not entirely sure what you're talking about. It certainly isn't because I want to go from threaded to threadless, I just want a different stem, one that doesn't angle down but goes straight out. Maybe, that's all I need to look for but stems are built differently these days so I thought that I needed to replace the whole kit-and-kaboodle.

There should still be plenty of options available with "Old School" quill stems and bars. About half of my OLD quills sit level, and the other half slightly down. Going to depend on the angle of your headtube, but lots of options available. Need to figure out the Diameter needed, Height needed, angle needed, and reach (how long FORE WARD)

I'm not so concerned about getting a perfectly level stem so I don't need to worry about the angle of the headtube. If I can find a stem that is close to 90degrees, I'd be happy. I think the reach is good on the one I have so I can measure it and use that as a basis.

Any suggestions as to where I might find quill stems? I'll look but if you have a good supplier, I'd like to hear it.

OK, so know I know a little more about what I need and what is available, I have learned that there are two options available to me: there are quite a number of replacement quill stems including some angle-adjustable ones and also there are quill converter posts that would enable me to use a "modern" stem and handle bar. I just need to find out the diameter of the quill stem, it appears that it is likely 25.4mm but I'll pull it off and check it out.

90% sure you have a 22.2mm quill and probably a 25.4 clamp diameter (but possibly a 26mm clamp, depends on the year of your Strada LX) and that clamp diameter is the fit issue for the handlebars: they must match OR you can shim a smaller (25.4) bar to fit the larger clamp D of a 26mm stem, but not vice-versa.
The angle of you current stem is almost certainly approx. -17º if original stock, SO THAT the top "leg" of the stem will be approx. parallel to the GROUND when installed in the bicycle...it would look to be "down-sloping" when viewed off the bike with the quill held straight (at 90º to the ground).
IF you want a stem with 0º angle (stem legs forming a 90º angle) your effective stem angle when installed will be +17º or so, lifting UP the handlebars and moving them slightly backward.
Many people desire this effect to improve fit and comfort.
Look for 1" quill stem (22.2mm quill diameter) with proper clamp diameter (25.4, 25.8 or 26mm) to match the bars you will use and the correct length (60-130mm range) to place the bars where you want them fore/aft. Often this style/size stem was common on early (1980s) Mountain bikes and the smaller clamp diameter (25.4) was typical as straight MTB bars were typical.
It might be called a "zero angle" stem or a "90 degree" stem or a "+17º stem" or ????, there's no science to the naming, so look at a picture or in person to make sure it's the angle you want.