If you are building a brass era Speedster and you have the freedom to shape things as you like then I have some basic ideas that were helpful to me.
Gas tank size. A good starting point for size if you have a barrel shaped tank behind the seats is half the outside diameter of a tyre.
Seat height. The amount the seat is lifted off the chassis rails is related to the wheelbase. What will look right on one wheelbase will look out of place on another.
General "lines". Even with all the parts seeming to be quite unrelated on a brass speedster you can still give the car some over all "line". Think of a line starting at the radiator cap, coming along the hood, then in a gentle curve, passing the centre of the steering wheel, top of the seats, top of the gas tank and down the spare tyres.
Where you can have most variation is the rear end. Too boxy and it can look homemade and rear heavy. Check out the rear of a Mercer Raceabout for some style.
Fender lines. Heaps of variation to be had here..... Straight like a Simplex, curved and coming hard down to the running board like a Bearcat, angling down like a Mercer...... curved over the wheels, or straight out..... artistic flick at the ends of the rears. Running boards or a step? No fenders at all!
The sky is the limit with fenders as a way to change your look.
Scuttle/dash hood or nothing? Depends on the years you are aiming for.... their are literally hundreds of ways of shaping a small dash hood that can look nice.
Seats. The seats and upholstry are a big parts of the look of a basic Speedster and can be shaped any number of ways, some nice brass or wood trims can add a touch of class.
Their are just some random thoughts! Speedcars are a perfect way be a little creative..... after years of slavishly restoring cars to factory spec it can be a lot of fun to do!