The following are excerpts from www.iwheeltravel.com: Penang Hill is one of the most accessible tourist places we visited in Malaysia. To get up there, you will need to board the funicular railway. Good news : this railway is free for disabled people (whatever is your nationality) whereas it will cost a non-Malaysian adult 30 MYR for a return ticket. There is no issue to get to the funicular's door, as you will find a wheelchair friendly gate and the staff will help you until you are inside the cabin. This help is necessary since there is an uneven ground between the cabin and the platform.
Once at the top, near the viewpoint, there is an accessible toilet, unfortunately it is impossible to transfer sideway from the wheelchair to use it. As for the food, well I didn't see any ramp nor access for wheelchair to go to the restaurant. So you can only use the two-level foodcourt. The bottom one seems not accessible, but the top one is.
According to Lonely Planet, it is possible to walk on the road downhill from there to the botanic garden. Only 5 km and 1h30 estimated duration seemed fine to me, I told to myself I could do it. Well no, this road is way too steep for a wheelchair user. There are pickup trucks that can take you to the garden for 80 MYR per person, but there is no accessible one.