You will be able to use a '73-81 type piston, just needs modifying to have the reed holes etc to match the original.
I think Boyeson dual stage reeds are available, a new set of reeds can't hurt especially if you don't know how old they are, else go for the v-force upgrade, other posts are probably on this forum about them as the yamaha and honda guys are using them.
Try Andre's site for the piston :
http://www.enduroklassiker.at/if Andre doesn't have what you need, contact Al Beuhner :
http://www.pentonpartsusa.com/2A%20KTM%20engine.htmelse contact me, and ill sort a piston kit out for you so its ready to go.