frame - if you can't source... i remember years back using a late sixties hodaka ace 100 frame to substitute for one on a recommendation and it being super close.... now how accurate that statement is i'll leave up to you to research... it very well might have needed some hackin/welding
swingarm - see above
tank - assume you saw the 'NOS derbi coppa FDT tank' on treats? if you don't wanna cold-call folks in spain they pop up on ebay occasionally. the laguna on there now ('Used Vintage Derby Laguna Sport Gas Fuel Tank Gray Army Moped Tank WT91') might be rough/expensive but hey maybe the dude will take an offer? at least its pretty dent-free... then there are expensive spanish tanks ("DERBI RED FUEL TANK DERBY (box 64)" and "DERBI 49 FDX DERBY SPORT NEW FUEL TANK")
seat - i feel like i just used a tomos one. there are reproductions all over. the oem shit is expensive (ie Derbi C 5 original new seat )