I'm not sure how the factory did these later Model 1894's.
If the sight was silver brazed ("Soldered") on re-attaching by using silver braze will ruin the finish requiring a refinish.
Real silver braze melts at temps above 1100 degrees and this destroys bluing.
You could reattach the ramp with soft solder which melts at temps below 450 degrees.
You have to be VERY careful to keep the flux off the bluing since it will also destroy it wherever it comes in contact with blue.
To soft solder you have to remove any bluing from the area where the sight is going to be attached, but if the ramp was brazed or soft soldered, this should already be done.
I recommend using a solder/paste mix to "sweat" the ramp on.
Brownell's 70-PA paste soft solder is good:
http://www.brownells.com/aspx/ns/store/productdetail.aspx?p=645
Do do this, you apply a dab of the paste to the ramp, position in place and heat with a torch until the solder melts.
Done carefully, you won't damage the bluing.
Another option that just might work well is the new "Black MAX" Loctite bonder Brownell's is selling.
This is a "super glue" type stuff mixed with a rubbery bonder. This apparently will permanently bond a ramp on a gun.
Personally, I think I'd be tempted to try the Black MAX first.
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=6139&title=BLACK+MAX+ADHESIVE
Another option, is to have the barrel drilled and tapped for a screw-on ramp.