Not many charcoal powered motorcycles out there wich l think is a real shame! If you are serious about this, l am first in line willing to help you on the project. I love the concept l have had a 50cc moped on chargas and l just loved it.
I have a feeling Harley engine might be a wery good candidate for chargas.
As much as l love the updraft gasifier for its simplicity, years of testing out different designs lead me to the design l currently use, a downdraft hybrid of the old WW2 style gasifiers and the WK gasifier. It gave me best performance of all with a minimal size package. It is more complex thugh.
Anyway, if l wuld be building, l wuld make something like this:
This is looking from behind. With this setup you are looking at a range of about 60 miles with one hopper filling + reserve char. Thats a rough guess with 12x12x25" hopper.