I modified my Cannondale hardtail with an Amped kit (geared rear hub, 48v / 20ah) . It really wasn't that difficult. I think the biggest task of the process was modifying my Blackburn rear rack to hold the battery. It's not super clean looking due to some extra wires/connectors. But on casual glance it doesn't look like an e-bike (and if I ever get around to hiding the battery in a rack bag, it'll be pretty stealthy).

Top speed w/ battery only is around 24mph on level ground. Pedalling will add a few mph to the top end, but the geared hub doesn't allow a very small sprocket (and I'm too cheap/lazy to add a bigger chainring). Honestly though, you wouldn't want to go much faster with an old hardtail (no suspension, no disk brakes, etc.)