I searched a little online and couldn't find where anyone had done this before. I did find where someone upgraded to 65c02 and had to replace a couple of chips to get the timing right.
Anyway, Apple never had an "enhancement" kit for the II? Could you upgrade the CPU and replace the ROMs with //e ROMs and get it to work? Nobody every tried that? 
There are hardware changes in a IIe that are not present in a II+, however you can run a 65C02 in a II+ but not all applications will work. The IIe ROM set won't work in a II+ because the hardware on the IIe motherboard has changed significantly.
I did it back in the day. I had a GTE 65C02 in a ][+, along with modified ROM images which had some of the Enhanced //e updates and a different 2716 in the CHARGEN, as well as some other hacks to make it more compatible with the //e. I was working on but never finished an 80 column card that would support the split memory the way a //e does it instead of a 6545 chip like most ][+ 80 column cards. In addition to cards in slots 0 and 3 it required running jumpers to several chips on the ][+ mobo to grab signals that were needed to simulate the AUX slot. Ultimately I eventually bought a used //e and lost interest in the project. Unfortunately I don't think I have any of the proto boards or my notes about it anymore either.
Anyway, as others have mentioned, you can't just use the Enhanced //e ROM images directly. What you have to do is start with the ][+ images and selectively replace just things like the parts of the Applesoft interpreter that are changed on the //e for example to allow lower case input, etc. It is a pretty complex part of it, and a lot of trial and error to make something work. These days it would be a lot easier since you could use an Emulator to get a lot of it working before actually burning EPROMs. Also you could use EEPROMs to save time.
Also I should mention that there are several different versions of the 65C02. GTE, Rockwell, WDC, etc. Some may work in a ][+ w/o additional mods or chip swapping, others may not.
I 'sort of' did it back in the day...
I took an Apple IIe keyboard and cut it into an Apple II+ case. I used a Videx Keyboard enhancer and did a few mods to the program ROM to incorporate the IIe Keyboard keys to output the proper ASCII codes for the II+.
If you look at this photo you will see my result. You may also notice a 'slot 8' addition on the left hand side of the motherboard. Function of this slot was primarily to power a Taxan Monitor RGB card for what was (at the time) my color monitor. I wired the appropriate signals & power to the card from underneath the motherboard.
Here is a photo of the underside of the keyboard. You will note that I did a lot of customization at the time to make this work. Still works well to this day.
To gain upper/lower case, a simple change to the F8 ROM (*FD7E:EA EA EA EA EA EA) and replace your CharGen IC with a Lower Case Character chip were also needed. But it worked out great
Nice, thanks everyone.
Will the II run a 65c02 with the stock ROMs or does it crash? I guess it should work unless it uses "illegal" opcodes. Also, would ProDOS 2.0 work or does it still rely on the extra 64k RAM in the IIe enhanced also...