Hi,
Re. front sight, what sort of range are you shooting over?...is the barrel bent upward?...is the barrel catch tight with no play? ...or any play in the action to stock joint?
Re. spares, these use leather piston and breech seals (can't remember the sizes, sorry)...Protek usually have Gem pattern springs in stock, so always worth giving them a call.
Edit: from memory...maybe a 1" cup seal for the piston seal and around 9-10mm flat seal approx 2-3mm thick for the breech seal???...please do check these sizes before you order anything!...and if you do get stuck I can make them for you if you verify the sizes needed.