Hope this advice is not too late...................You said in your intro car has lay idle for 15 years (just like mine in pic on your left), but you can't get it to start.
When I found my GTV6 165HP D Port "Wild Plum" sad and alone in its garage for many years in Central Victoria, the first thing I knew to do, as with ANY engine that hasn't "turned over" in 6 months or more, is to remove ALL spark plugs and squirt in a few tablespoons of "LUBE" in each pot (Lube can be 50/50 WD40/10w30 oil, or 100% WD40 if you suspect internal rust/corrosion, even 100% diesel fuel is very good and my personal favourite)........let this soak for 5 mins to 1 hour; pref 1 hr if rust/corrosion is suspected; then add more. With ring spanner on Crank Front Bolt, try moving crank back and forth 1 degree at a time to free up any chance of rings sticking and then breaking, until you get to a full revolution or 2 of the crank.........if moderate force doesn't budge the crank, STOP! and add 2-3 more tablespoons of "lube" and let sit overnight........then try again next day. 99/100 times you'll be ok and able to incrementally move crank as above. Once you get 2 or 3 full revolutions and it's feeling free, hit the starter for 5 seconds to expel excess "lube"....Remove rocker covers and squirt new oil all over valves and rockers and push-rods. Don't put covers back yet..... Install Sp/Plugs....... Run engine till you see oil squirting at rockers & valves.... Stop engine and fit Covers. Run engine till warm and then change oil with a new filter.......time well spent if it avoids a ruined $5,000- $25,000 engine.......Enjoy......NB.........First ensure the subject engine is safe to turn ACW lest the Timing Chain &/or Tensioner should become dislodged.........if so, only turn CW a degree at a time..........My Capri got this "love" and she re-paid me by starting and running beautifully, and I then drove it hundreds of kilometres back home, purring like a kitten, if I ignore the slightly "burnt" valve in one pot and the noisy water-pump that stopped leaking as soon as it hit the highway!...........I also topped up the stale old fuel with 10 litres of "Shell" 98 VeePower which worked wonders !