That chassis doesn't have a motor mount as such. The motor is just clamped between the rear bulkhead and the battery compartment with the aid of a locating plastic ring on the rear bushing/bearing housing of the motor can and held against counter rotation by a screw head on the front of the can that locates in a little recess in the rear bulkhead. Modern B/L motors would have the locationg screw hole in the right place, but the rear end of the can is totally different as it doesn't have the protruding bearing housing. That will be quite a challenge to overcome.
Apart from that, I wouldn't risk destroying the gears with a brushless in that car. Pinion choice is limited to one (or two, I think you can have a 16 or 18 tooth pinion with associated plastic gears) if I remember correctly. This hardly gives you the right pinion choice for brushless so getting the right gear will be a choice of motors rather than gears.
But apart from all that, the car is not really competitive. It is fragile and limited tuning-wise. The suspension is a joke and there is no way to tune diff action. I remember I broke one front gear housing straight out of the box with a low speed impact (where it attches to the battery compartment, it is held in place with only one screw and a locking aluminium tab. This is really pathetic as teh entire frnt end of the car is attached to the rest at this joint.)
Finding bearings for them is not that difficult ( a must for any high power application), but you will need a number (two or four, can't remember exactly) of really small bearings (2x6x3 IIRC) and these are found in some slot cars which makes them a bit tricky to get hold of.
Don't get me wrong, I love these cars and I have restored two already but I rarely take them out and then only for dirt tracks where there's nothing to hit. One of my cars even has the centre differential option installed but that again is just for curiosity as on the dirt track it is just useless (as soon as you hang one wheel, you lose traction). Locked diffs would make a lot more sense. On road, there is no choice of tires as the wheels hexes only fit the Tamiya wheels and these only have one tire option for offroad and one for on-road. All these being discontinued a long time ago, whatever old stock you find will be aged and hard.
Have fun with it, it's a great little oddity with incredible charm, but racer it ain't. You might however want to consider restoring it as a shelf queen in which case I strongly suggest you start looking for parts. Since I restored my cars (a few years ago), I have noticed that part stocks are seriously depleted in all the usual places. This might mean they could fetch a nice price even as a source of spares but there is only one way to check that one out and I am definitely not selling anything. the only good news is that there are a bunch of companies floating about which are making repro Porsche 959 bodyshells and they're getting better with each iteration. If your shell is in good shape (i.e. no cracks, bits broken off) you can still salvage and restore it to concours condition but it ain't going to be easy. Remember, that's probably the most valuable bit of the car, together with an original Rothmans decal set.
Team Greasy Weasel
The best upgrade to any car is some driver skill.