Better late than never.
You can do an engine swap with an E series engine from Daihatsu. Try getting a Daihatsu Terios Kid engine EF-DET 660cc. Also comes in the following cars as far as I know:
Daihatsu Cuore TR-XX
Engine is a 660 cc Turbo Engine with a top mount intercooler and a power output equivalent to that of the Duet, with slightly better fuel consumption.
Or if you don't want a turbo engine, go for 850cc ED10 engine from our local Cuore. Pretty easy to find them, fuel average is also decent. Power is low at 44 BHP (Duet has 64) but hey - parts are easy to come by. Personally, I'd like to have the turbo.
Why Daihatsu for a Toyota? Because that Duet is just a rebadged Daihatsu Storia.
Hope it helps.