Ryan, this is terrible news. It seems like we've had a spat of engine's going out recently. Like you, I was very faithful on oil changes and never really dogged the engine (I rarely reved it past 3500 rpm's), but I guess some of them are just going to give up the ghost no matter what.
Of the two options you listed, I would go with the full rebuild if you have the dough. Make SURE that the new pistons have the extra oil drain holes in them or you'll end up with an oil burning issue again (if you had one in the first place). Texon looks like a good company and the Isuzu engine is just an engine so if they're business is rebuilding engines then everything should be just fine. Do you know what is wrong with your engine? I'm just afraid that they will get into it to rebuild it and find something that they can't fix so you'll be out of a useable VX AND the money that it took to get into the motor to find out it's unfixable. I'm usually an optimist, but that was my biggest concern when my motor needed to be replaced. What if I spend all this time and money to have the engine removed and broken down only to find that it can't be rebuilt then I have to spend more money for a new or rebuilt motor AND the VX is out of commission for an undetermined amount of time (it's my daily driver).
If money was no object, or you could take out a loan for it, I would totally endorse buying a new Isuzu motor. When I got mine there were only 16 left in the country so I have no idea if there are many left. The motor that I bought was, apparently, manufactured in 2011 and has the updated pistons with the extra oil drain holes in them. I've been driving on the new motor for over 4,000 miles now and I've only had to add a tiny bit of oil after about 3,000 miles (I used to have to add 3/4 of a quart every 300-350 miles). I'd be happy to get you in touch with Jerry Lemond as he can get a little bit of a discount on new Isuzu motors.
Good luck.
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