Is this a straight-forward swap with the bearbox as well?
We don't tend to get involved with advice on them here...
I am unsure if you mean you think the box is starting to pack up or if it's just the change selection mechanism..
Very often the selectors get worn, or lose their adjustment, there is a 'lot' of info in the Wiki on this, so read all this stuff first and see if it makes sense, then get back here with any further questions... its a complicated system of levers and pivots and joints.
I expect it is that causing the problems changing gear, although at least check that the box itself is not leaking oil at the joints between the castings... if there's any sign at all, then check the oil level in the box asap, even change it just for peace of mind (about 4 litres I think). The magnetic drain plug will give you a good idea of the condition, by what sort of fir-tree you find on it, or any larger pieces that might come out.. keep them as evidence!
As advice above, the 1.9 ND (1Y) is an easier conversion, the TD requires more work, has more to go wrong and when installed they produce more heat and often benefit from an oi cooler up-front, which isn't exactly cheap. They both fit fairly easily though onto your current gearbox, but both require a slight increase in gearing, best obtained by simply increasing wheel size to 15" with suitable Aspect Ratio tyres that give a 5~10% larger rolling circumference.
Once converted, you can then judge whether you really need a different gearbox, but it's really the TDi conversions that need much higher gears, not necessarily the NA and TD IDI engines.
I would though, put a completely new clucth in at point of conversion, clean and degrease the flywheel surface well, and use a new spigot bearing, also checking that the clutch arm and its bearing in the bell-housing are in good condition and not stiff as well as grinding any burrs off the operating fork ends, to give a nice smooth clutch action and assist gearchanges.
PS. If possible, try to hear the donor engine running, preferably see it start from cold too! If it has maintained well with regular air, oil and fuel filter changes and oil changes, they should last at least 150k miles.
