The Red Lion sight has windage and elevation adjustments, so you just line it up as close as you can visually before you lock it down. If you mount a factory Kel-tec forearm it will give you a decent baseline for eyeballing the front sight. The Kel-tec rear peep sight can be re-installed without needing to split the receiver halves. You'll just need to remove the upper screws, sleeve, and barrel assembly to replace it. I have removed any reference to the part you want to sell as we do not allow sales or trade in the general forums. Please have a look at our forum rules linked below. Welcome to the forum.
ETA: It will be less expensive to call Kel-tec and order the rear sight. Webstore orders have a UPS minimum shipping charge. Phone orders just pay postage normally.