It is the old "How long is a piece of string?" issue and ultimately it will be up to a couple of factors and your own intuition:
1. Are you prepared to spend the money in the first place(you have to admit the old mini sucks up a lot of funds...); and
2. Mitigate the risk and re-mate your block back on your gearbox only having to rip it back out in 12 months to rebuild it anyway. This is a decision only you can make but be prepared to accept a more busted engine in the worst case scenario if you go this track.
It is never an easy decision. For example I have a Leyland that has gone around the clock twice on the factory motor while I have a totally rebuilt engine sitting idly on my garage floor waiting for the working one to give up the ghost. I am itching to get it in but I also dont want to do it unnecessarily as this motor I have is sweet as a nut(even with the very high K's on it...).
I am not trying to be the purveyor of doom and gloom but IMHO the top bit of the motor should be at least checked thoroughly and given the OK before you put the thing back on the gearbox. If it wasnt blowing smoke then I would risk it but try and replace what you can easily(thrusts, main bearings and check all tolerances where you can) and replace any seals you can get to while you have it in this state. If it was working well before you separated it, I think you could be ok(my opinion only...).
Again it aint all bad - you may find it will not cause you any grief for years but remember Murphys Law - what can go wrong will go wrong(Murphy was a pessimist and I wouldnt trust him anyway!!)
Good luck with whatever you do mate and I hope it works out for you. REMEMBER!!!! to replace the oil pick-up O ring if nothing else - otherwise there will be no oil pressure and once the box and motor are back together, there is no way to get it back in there unless you separate the motor again. A 10 cent rubber o-ring will give you a coronary if you forget to put it back...lol
Hooroo
Rob Forsyth
Miniot!!