Much time, cutting body panels.... it won't phsically fit without hacking into the body shell. Aside from being a bit slower, you will gain nothing but headaches and an empty check book paying for repairs for said 914 engine. Just my experienced opinion. The upright engines, while lacking in the power category, are as a general rule, more reliable and easier, cheaper to fix. As a matter of fact, there are a number of people who have spent the money and taken the steps to put the earlier engines into the later type-4 powered buses.
I'm just saying..... I wouldn't do it, myself.