As said it’s a good idea to check road wheel size and also count the teeth on your difs. Most wheels are changed for aesthetics rather than performance and sometimes the gearing of the differentials changed in an attempt to improve top speed...
Just a quick query before you start worrying about the engine, what size tyres are you running? The diesel is ideally suited to 6.00/16 tyres Many people fit 7.50/16s which can leave the diesel feeling slightly over-geared, trading off even worse...
Which Isuzu engine? I have heard of people fitting a 2.8 Isuzu turbodiesel to Series 3's but it is not a particularly common conversion. Those Isuzu engines are very reliable and can do huge mileages so unless there is something wrong with the...