Give it up. Yeah, I know Cam- diesel gets a bad rap in this country. In europe, where fuel costs (through taxes) have driven automobile efficency to a much greater extent than government mandates, diesel makes up something like 40% of all new vehicles sold. In the land where $2 fuel about causes a revolution- the economy of a diesel fades. Due to the particulate emmissions of older diesels, some enviornmentalists are leary at best, and hostile at worst. Modern diesel should be the future, but it never will be- it's a more economical, better solution than a hybred.
That said, a speedster is a bad application. Think of it this way- Most guys don't drive their cars 5000 mi a year. At 25 mpg, they will burn (at the most) 200 gal of fuel a year. At $2/ gal thats $400 for fuel- tops. Lets say you could find a diesel to fit your speedster (you couldn't), and adapt it to your car (again- I think not)- even if the fuel was free (french fry oil, etc.), ther would still be a phenominal amount of work involved for the pre-heating of the oil (its gelatenous at room temperature) even if you had the space in the car(you don't).
Bio-deisel is unweildy and largely impractical even for people who drive trucks 100K mi/ yr, and own a french-fry and fried chicken emporium- because of the modificatios required WHEN A DIESEL ENGINE IS ALREADY IN THE TRUCK.
A cold fusion powerplant, or a jet engine, or a super-conducter rail set-up would be neat too. We can't even engineer good heat in these cars, and you are trying to re-invent the wheel. You want to tinker with the idea of a diesel convertible? Buy a deisel Rabbit and cut the top off.