I plan to use the Victrix Limited plastic "Napoleon's Old Guard Grenadiers" for the bodies and arms of my conscripts. I'll do my best to try to replace the weapons with some spare Lasguns to give them something of a futuristic look. This will likely prove difficult, however, because of how some of the figures are holding the muskets, and because the hands and arms are typically closer together on these figures than on Games Workshop ones, making it difficult for them to hold the Lasguns.
For heads, I bought a box of the Perry Miniatures plastic "French Napoleonic Hussars". I can eventually convert the figures into Rough Riders for my Renegade Guard army, and the set comes with tons of head options, which results in having plenty of heads left over to outfit most or all of my Conscripts. They won't all have exactly the same headgear, but I think they'll look similar enough, and I can always imagine that the big block of conscripts has different squads issued different hats for whatever reason.
Here are some pictures of my first few Conscript conversions. Three of these are sporting three different head varieties from the Hussars set. The other two have heads left over from other things. The Lasguns are old 2nd Edition Eldar Lasguns. They're working out okay, though the hands look big on these models, but I can just say they're wearing really bulky gloves.
As you can see, for a couple of the figures I just replaced the end of the musket barrel with the end of a Lasgun barrel to make a Las-musket. I thought it would have looked strange or been very difficult, because of how those figures were holding the weapons, if I tried to replace them with full Lasguns. But I could see a unit of conscripts having different weapons, being issued with whatever happened to be available. In general, though, I think converting these guys to be holding Lasguns is going to be pretty difficult, and I may opt for more Las-muskets.
One thing I haven't decided on yet is whether or not I want to use the backpacks from the Old Guard set. I have some pictures of what a figure looks like with and without the backpack, but I can't decide which I prefer. Any opinions?