High speed pic scaler for Survey meter for PIC16F627A translation to PIC18F2855?

[Seoullasers]

Hello this will be my first post on the forum. I am an Educator from Canada whom has spent nearly 7 years working and living abroad in ROK and Japan.
I came back to Canada almost 2.5 years ago and I am re-adapting to life here.




I think I have an interesting predicament. I have an old code that I'd like to translate for use on a PIC18F2855. The problem is that not only does the code need translating from the PIC16F627A it also needs to be able to drive a 16x2 1602 display.
I have been in touch with the creator of the code and he has given me full permissions to modify and use non-commercially. After I figure out what I need to do I will give him the code.

Purpose of the code.

I have a project that I am doing. This is modernizing a Survey Meter (Bicron Surveyor M) takes (GM tubes, proportional detectors as well as a multitude of various kinds of Scintillation detectors).
The code in the .doc is for pulling in pulses and totalizing them at high speed (down to ~4uS). The program can take samples at CPS, CPM and a user defined count time (0.1/1/10/240..etc) windows.
The data is displayed on an LCD screen and also sent over a simple serial connection (9600baud). That's all this program is designed to do essentially.

I would REALLY like to adapt this code to make it run on a PIC18F2855. I recently got ahold of one of these PIC18F's as a pre assembled kit called a (ECIO28P).
I haven't had much luck getting this code to adapt ( I've tried a number of changes and none has worked yet) . Additionally I'd like to be able to drive this on a 2x16 1602 (not over I2C) LCD screen if possible.

I have attached a commented release of the code if this helps any. I am quite lost.

**If there is anyone who would be able to have a look at the code and make suggestions or edits or show me what
would need to be done to the code that'd be a major/ serious help.**

Thanks.