you can try a hotter thermostat but i'd check a couple things first

- make sure the bypass circuit on the cooling system isnt blocked, if its clogged up and not circulating properly it will make the engine take a long time to get hot.
- does it have an add-on heater system on it? this will make it take longer to heat up also, more coolant to deal with and if fan is running on heater that will hinder the coolant heating up too. (most of the time these heater units are tied into the bypass circuit)

:o)