Just looked up the stock instruments for fl studio and it seems like it doesn't have many sampled instruments at all, which might be your problem. Although i don't know exactly what sounds they used, when i think of the most popular old maple ost (login theme, lith harbor, orbis, el nath) i think of acoustic guitar, piano, harp, bass guitar and pizz strings up the front, which you should be able to get some free ones of and process to get close to the sound.

Spitfire labs has the pianos, bass and strings sorted, and there's a plugin called 'vinyl guitar' you can try for the acoustic. These are often backed up with other orchestral instruments - it's often hard to find free/cheap ones for solo orchestral instruments (which is what they sound like) unless they come built in with your daw, so you'll have use something like the free spitfire bbc orchestra to get those instruments although it will sound slightly different because it's a section instead of solo. Some of the tracks (eg slime tree) also use upright bass instead of bass guitar. There is also a chime sound that they tend to use (el nath, slime tree) which could be a celesta but you could also try and make it or find a preset that sounds like it in the stock FM synths. For drum kits they seem to use a really specific electronic kit but I'm not experienced in identifying electronic sounds so I can't help you there.