Old Jimmy to a new one......

I wouldnt lose heart on getting a new one, at list. 

There are still some from this years batch to arrive at dealers in September/October and there are 400 more scheduled for next year. There are dealers that sell at list and also do not have waiting lists. 

This guy on YouTube started doing one, a lot of work involved, not done a new video since last October.

I know it's relatively simple ish to transfer the front of the later gen3 onto an earlier Jimny but given the lack of common parts between the 3 and the 4 it seems like a lot of time and effort. With 12k I would be after a late 3 and putting some tasty modifications on it but keeping it true to what it is.

The easiest option would be dropping a Gen4 body on a Gen3 chassis. There were very few significant changes to the chassis and driveline between the models. I wouldn't even be suprised if the body mounts lined up.
But getting a Gen4 body won't be cheap.
And in the big scheme of things it's a big waste of time just to make a dodgy version of what is still a relatively cheap car. The Gen3 is a great car in its own right, with just as much charm as the Gen4. Your budget will go a long way towards customising one and making it your own. Surely something original is better than a bad copy?

You guys are amazing thanks for the great advice! I figured as much. I have thought of other alternatives but it is the smaller engine size and size it’s self of the car that appealed to me. (I’ll happily to admit driving anything bigger will just end me in never reversing into a space again! Haha - women don’t do spacial awareness) I might just hold out a few more weeks a see what the my local dealer says. They’ve agreed to let me know as soon as the two they are getting come in, so we will see. I may call around some not so local and see if they will be willing to put me down.  If not I maybe messaging some of you for a rebuild. I get what some of you are saying but it’s the gen 4 that I liked the look of, the gen 3 just looks……outdated to me  Don’t judge, I’m a millennial 

Well, the only thing to do in that case is to get your name on the waiting list for a Gen 4 Commercial.

You will likely have to wait a while for delivery, but that will give you chance to save up the £20,000 you'll need to buy it.