Yep, WYL is right as regards the green (treated) woodchip and adhesive.
2 options !!
1, as WYL says, ply over and with 6mm minimum ply using ringshank nails then glue the floor down.
2, My preferred option. whether the woodchip is standard or treated. Cover with Novia building paper
www.novia.co.uk it costs just £1.00 per/m2 and comes in 25m/2 rolls, so you will need 4. and secret nail the floor down. You get a much better fix and lose the additional height and cost the ply would give.
It also gives you a tighter fixing between the boards so if there are any bits of old adhesive you've missed the action of the secret nailer will pull the boards together. you can hire a "portanailer" from Hss Hire.
If it's reclaimed maple i imagine you will sand it after installation, if so, keep the sanding dust and mix with Lecol to use as a filler for any gaps/damaged boards before finishing with varnish...
The only way to remove the old adhesive is with a new paint scraper or an old (not so sharp) chisel i'm afraid, unless anyone has thought of a better way? The glue dres brittle and comes off fairly easy, use a new paint scraper for the tongue and a chilsel that fits snuggly into the groove (approx 6mm). You could also use a router for both but a bit tricky DIY.
75m/2 will take a while but will be worth it.