As BlackKnight says, there's broad compatibility with any game based off D&D. I've used AD&D, Basic, OSE, Blueholme, C&C, Adventures Dark and Deep, and DCC stuff at various points. Hyperborea itself is Holmes Basic + AD&D, leaning a bit more on AD&D I'd say, so that's probably the easiest source to adapt from. I'd recommend grabbing OSRIC (1e clone) and For Gold & Glory (2e clone), they're both free.
Spells should be pretty straight forward. Classes will take a bit of work, there's a slight difference in approach to class design between AD&D and Hyperborea (compare the UA barbarian and cavalier to Hyperborea's barbarian and cataphract for a pretty stark example) so there's a bit more art than science to converting those over; I'd probably start by considering if the class you want would be easier to create by making a couple modifications to an existing class (a samurai is pretty much a cataphract, except the 1st level weapon mastery should be long bow and bastard sword and they shouldn't have access to shields, so replace the shield sacrifice ability with something else).
Psionics is definitely trickier, it's something I've put some thought into myself. 1e psionics is pretty messy, as a system, while 2e has two different sets of rules that have similarities but are not fully compatible, one in the Complete Psionics Handbook and one in Player's Options: Skills & Powers. There's another option I'm kinda taken with, which is using the psionics rules from Gamma World. I saw someone explain how they did that in a comment on a Grognardia article, basically use the AD&D 1e rules for determining if someone has psionics, then roll on the Gamma World table, rerolling negative abilities. You can grab the art-less version of Mutant Future for this purpose too.