Quickest, cheapest and easiest is to find another 1.3 and swap it out. These little Samurai are probably the easiest vehicle to wrench on that was ever built. Honestly, the last engine install I did took three hours, from the engine coming off the stand, to starting it up and backing it out of the shop. You'd think the little truck was designed by a mechanic, because everything is so easily accessible.
I'd be looking on Craigslist for a running replacement. Should you decide to freshen it up, before putting it in, the 1.3 very simple, is easy to work on and parts are cheap. The shop manual is available online, for free.
If you decide you don't want to repair it, what do you want for it? I've got three Samurai and can always use another one.
As for cost, I've seen running 1.3s for as little as $300. As for doing a full rebuild, you can spend anywhere from $250 for a basic rebuild kit, to whatever, but you're still going to need a replacement block for your ventilated one.