Im very pro-DIY because you always cone out of it with something valuable whether the thing you build is actually good or not! Ive never ‘failed to get better’ in the act of building something.
I own a B6100 and B7100 and agree they are fantastic for tight spaces.
So ive got a few ideas. For one thing, lots of stuff the size of golf carts or Japanese Kei trucks for example could be made into a pretty good SCUT replacement with enough gear reduction, and added weight to create traction if limited to 2wd. So if you can find a shell of either of those it would be a decent starting point and save you a ton of hassle figuring out the steering system and possibly the brakes. Even an electric golf/utility cart is a perfectly decent starting point for a gas engine ‘b6100 replacement’ project, and thats coming from a guy that owns a B6100 and a couple of golf carts. Not sure about availability of either thing in your area, though.
If building from scratch, another thing to consider is taking a fwd car transaxle, and mounting it longitudinally between two truck axles (narrowed preferably although i understand its far easier said than done). Doing this gets you ‘full time awd with center differential’ so it will turn sharp nicely but the real point is that between the fwd transaxle’s 1st gear and ring and pinion ratios it will be lower geared than the low range in most 4wd trucks, potentially FAR lower, before it ever gets to your drive axles. The differential in the transaxle could be welded if you wanted a ‘locked center diff’ or if you wanted to drive only one axle from it. A fwd transxle with a welded differential and a single modified cv axle feeding a 2wd truck axle, is potentially a very compact and cheap setup.
If you decide to use a small aircooled engine (which i probably would even though the car parts are such glorious overkill) you may consider the idea that as long as you have enough gear reduction, you don’t really need a ton of clutch strength and COULD get by with a centrifugal clutch or ‘torque converter’ as used on go karts. There are videos of tractors and full size trucks running off of centrifugal clutches on youtube.. although a torque converter ‘belt cvt’ would be preferable. Saves you the hassle of building (and rebuilding, and adjusting, and then repairing) a clutch linkage.
Also, one potentially very simple way to get drive power AND pto power is to use a vertical shaft riding mower engine with two pullies aready on it, a fixed drive pulley and an electric pto clutch. You could use two ‘mule drive’ belt setups to feed your trans and pto. Mule drives were used for cutting decks and snowblowers on garden tractors for many years. You add a few idler pulleys and some bracketry but it lets you rotate and translate (ie move sideways) the power coming out of the engine 90 degrees very cheaply compared to a lot of potential gearboxes and shafts arrangements, or hydraulics.