Midland gearbox revision/exchange

Hello dear forum community have n R50 Cooper with 5 gear gears here engine is chipped and brings according to performance report round 140 hp. The gears is a Midland , this kracht very strong in reverse gears otherwise it does not make any problems up to now. Basically the first question I ask myself is it is possible to install the gear 5 gears into my mini or does not fit that ? Unfortunately could not find any concrete information about it yet. The other possibility w of course to order a gear kit in England, i.e. the replacement shafts, seems to be everything except the 5 and the reverse gear, are these absolutely necessary? If yes, where? Is there a concrete instruction or other for the gear or the shaft/gear change? I am happy about the answers and hope there are not too many questions