Hello giovanni

It depends on what you mean by 'outside'. If under a roof, with no water or snow, you could possibly drape it with a heavy tarp (or have tarp material glued or otherwise attached to the sides hanging to the ground like a skirt to reduce air circulating out of the engine bay - depending on circumstances) and use an electic fan heater on low under the unit. Heat rises, and the fan circulates it.

The temperature can be set by the thermostat, however our experience with fan heaters is that it's best to set on max temp and select heat output by power position (there's usually 2 heat settings for the element).

If there's any water underneath, I would probably discourage this idea, for safety reasons. Better a hard to start engine than being electrocuted.

Another option is to use a Diesel Air Heater (plenty of them on eBay, see one here: https://www.ebay.com/itm/224268313444 , here's a controller: https://www.ebay.com/itm//254490256749) and pipe it outside - again, you will need to do some sealing with a tarp or skirts to allow the warm air to collect under the forklift without blowing away. There's a few options out there!

Another benefit of the above options is that the fuel system stays warmer too - making it likely easier to start.

Keep a sharp eye on it the first time or two until you have a feel for how your setup performs - you don't want to overheat it and set it alight...

Hope something works for you!

KR

Charles