Fungus Woods/Scenario Guide
Start slowly with the available flat\non-tracked rides. After you have enough cash, you can start building coasters. For every coaster, do a campaign for the coaster and the rest of the scenario will be easy cake.
The first coaster you should build is the Reverser Roller Coaster, due it's cheapness and getting easy high excitement rating if built properly. When your money starts flowing into your pocket as the guests comes to your park, build the Wooden Roller Coaster as you want and try it to have an excitement rating of at least 7. That will bring more guests, but campaigning the coaster will surely get your objective done.
The limit of the wooden rides only sounds pretty challenging, but beating the scenario is not hard as beating the Amity Airfield scenario.
Marcel Vos' tutorial 'How to beat Fungus Woods'