Hey all. So I recently bought a 2005 Hijet. It’s an automatic jumbo cab and admittedly I didn’t do the proper research before purchasing, However I am determined to find a solution. My question for you fine folk is: Could I swap a 4-cyl engine from a Subaru Sambar into my hijet without having to modify the frame or stance of the truck? I’m trying to get my truck to reliably do highway speeds (80km/h) so I can use it as my daily and so far the stock engine does not like doing highway speeds. Any insight is appreciated, thanks! 
Can you explain a bit more about it not liking doing highway speeds? What, specifically, is wrong?
Assuming its a KF-VE (658cc I3, Petrol) then you should have approx 50bhp and 60lb/ft available. Max power comes on at 7,000rpm - so in common with most/all daihatsu engines it needs to be revved - but this is not a bad thing.
80kph/50mph should be easily attainable? It might be revving at 3000-4000rpm (I’m not an expert on gearing on these, especially automatics) but this is absolutely fine for a little 3 cylinder Dai engine, and it will happily do this for many, many years. What is it doing exactly that makes you think its not happy about doing a sustained 80kph?
Vibrating? Overheating? Knocking?
What is the service history like? When was the oil last changed? What colour is it? When were the plugs last replaced? What about the oil filter and air filter?
I think putting a new engine in may not be the solution but it maybe other factors such as granger has eluded to also what are your diffs like and the gearbox? how are they geared?
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As a commercial purpose Dai it will have a really low final drive/diff and fairly short top gear.
It has Overheated on me 2 times now. First time was due to a rad issue i had fixed. Second time it blew the head gasket and is currently waiting on the part to come in to be fixed. However my mechanic has told me that this will likely happen again due to running the engine at high rpm for 45-1hr long trips. It also struggles uphill for the few I have in my area and based on what ive seen the supercharged sambar has no issues with most hills. Unless there is a way to change the gearing in the truck to make it more highway/hill friendly then the Sambar engine swap seems like my best option performance wise. I was just trying to figure out if it would fit without having to modify and custom fabricate the existing truck chassis before taking on the project.
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If you use the same gearbox, the Sambar engine will be revving just as high for sustained periods of time? I’d suggest looking at a different gearbox/different final drive, though be aware this will blunt acceleration significantly. A 5-speed manual gearbox would give you the best flexibility I suspect, though you’re always going to run into the harsh reality of the amount of power a 658cc three cylinder engine will deliver and the aerodynamic efficiency of a van body.