Regarding the topic of finding a new engine to replace the WLT's, I'm still hunting around. Rumor has it that that 3.0L Ranger engines are more reliable (meaning the head is stronger and more resistant to cracking). I don't know how true that is, but I am looking into it, once time permits.
I was also considering to work a Toyota 1KZ. I believe it's just to build over new engine mounts and probably an adapter plate to mate it to the transmission. Not sure of the electrical parts yet. But again, I haven't put much effort into it yet.
A friend also found a Mazda flatbed truck engine that suppose to fit as well. Lol. I heard people in Australia did a swap with it already (massive torque for off-road). Don't think for everyday use will be good though. Low RPM, low top speed.
Now I share your interest in finding a new engine (I want a diesel though) because of the engine head issues, but at the moment, I am trying to find a solution into preventing the engine head from cracking. I will have to make a thread and explain all the information I gathered so far, but for now, do this.
Go in Lenny Sumadh (not sure of the spelling). That's on Coffee Street, San Fernando. It's just after the funeral home (coming from Southern Food Basket Grocery side) on the same side. They have the Murphy Gauge (as 3stagevtec stated earlier) for around $550. If you search the forums, someone already posted about the gauge and I explained how to wire it up. Basically, it can switch off your engine, or if you prefer, turn on an LED light to notify you that your engine is running too hot. I connected mine to the heater line in my van with some simple couplings.
Also. Put back in your thermostat. It's actually doing more harm to your engine in my opinion. The thermostat regulates your temperature to keep it at a constant temperature. When you drive your engine hard for a while, then run it easy after, everyday, the temperature rises and drops everytime causing fatigue on the engine head. Expansion and contraction weakens aluminum. Also, running the engine too cold doesn't allow for the engine to expand to normal operating temperature, so the pistons and other components don't seal properly. I don't know if my theory is correct, but I rather play it safe. All you are doing is creating a big buffer which takes the engine a little longer to heat up when you get on the highway. It DOES NOT help keep the engine temperature down on the highway. It will rise to the same temperature as when the thermostat was in, just take a minute or two longer. I've done it already and monitored it with the Murphy gauge. My suggestion to you (which is the same thing I did), was drilled 3 small holes in the thermostat to allow the engine to run a little cooler. Reason for this is that I believe there is a hot spot in the engine (from examining the exhaust manifold). So I decided to run the engine temperature a slight bit cooler than normal for everyday driving around. Note that it's only on the highway, in the hot/mild sun and driving over 120km/h causes the engine to reach an average of 93degrees on the highway. Anything past 120km/h can cause it to reach and sometimes surpass 100degrees. In the night, it doesn't pass 95degrees even when you run it hard.
Another thing with the thermostat removed is your fuel efficiency. It drops like a brick. I can get 500km on a full tank if I drive efficiently, but with the thermostat removed, I struggle to reach around 430km. Your engine may be newer and better tuned (I have a B2500, and there are differences between mine and the 2008 Ranger, which is the same engine as your BT50), so you may get more distance on a tank, but it should suffer from similar efficiency loss. I also stutter and smoke when the engine is cold due to the removed thermostat.
3 holes drilled in my thermostat:
Note. The holes are only to keep the city running temperature a slight bit lower than the normal 83degrees operating temperature. This is to help with my theory of the hot spot in the engine by running it a little cooler. It does NOT help with highway driving. Also help with the slight flow increase you get with removing the thermostat. The thermostat doesn't restrict coolant flow that much when fully opened, but the 3 holes CAN help eliminate that slight restriction.