Have a search for this, I've written a few detailed replies alraeady.
In summary these days (and with an engine of that age you wil lbe "caught") all the inner workings of the remotes are identical, and if not mechanically identical will provide the same throw on the cable end. Net result is you can use pretty much anyone's lever mechanism to work throttle & gear. Can also buy Morse / teleflex / Ultraflex cables to fit them at about half the price of the branded ones. Cable end kits (with the fittings for the engine are between £5 - £20 depending on the engine.
My Merc merc is being controlled by a Yam 703. When I was rebuilding the 703, I borrowed a Tohatsu control box (Friend with Tohatsu had used a flush mount aftermarket lever for his engine). I also ran the Yam from the Tohatsu box..... Having had the 703 and my old Suz one apart, the insides of those are also identical .....
It's the electrical connections are the "unique" bit. If you want to "console mount" the switches I believe you can get "plug & play" wired switch panels for that sort of game. Worst case if you are on a budget, an engine breakers will be able to provide a random lever, then you are just buying the wiring new, which should be cheaper.
As Uncle Al says, any Merc / Mariner dealer should be able to help.