Here's how the records are stored.
The names are as simple as three ASCII characters. Just edit them on the decoded text area of your hex editor and you're good to go.
As for the times, they are expressed in milliseconds and are stored as integers, with Little endian as their byte order. So here's how to work with them.
For example, take the default 1st place record. Its value is 7dac03. The first thing you need to do is convert it from Little endian to Big endian. In other words, separate its bytes in this manner: 7d-ac-03, then invert its byte order, and the resulting value will be 03-ac-7d. Finally, just enter the value into any hexadecimal to decimal converter and you will get 240765, which is the record you're working with expressed in milliseconds. If expressed in the minutes-seconds-milliseconds format that the game uses, the value is 4'00"765.
And of course, if you want to modify that value and put it in your savefile, you need to follow these instructions in reverse order.
Finally, each car is identified by a number. Here's a list:
00: F/A Racing
01: RT Ryukyu
02: RT Yellow Solvalou
03: RT Blue Solvalou
04: RT Pink Mappy
05: RT Blue Mappy
06: Galaga RT Plid's
07: Galaga RT Carrot
08: RT Bosconian
09: RT Nebulasray
0a: RT Xevious Red
0b: RT Xevious Green
0c: 13th Racing
Again, the car identifier in the Time Trial Records is Little endian, but only the least significant byte (the first one) can ever be anything other than zero.

TIME TRIAL RECORDS (24-bit/3-bytes values)
0x180 Time Trial 1st Place Name
0x184 Time Trial 1st Place Total Time
0x188 Time Trial 1st Place Car
0x18c Time Trial 2nd Place Name
0x190 Time Trial 2nd Place Total Time
0x194 Time Trial 2nd Place Car
0x198 Time Trial 3rd Place Name
0x19c Time Trial 3rd Place Total Time
0x1a0 Time Trial 3rd Place Car
0x1a4 Time Trial 4th Place Name
0x1a8 Time Trial 4th Place Total Time
0x1ac Time Trial 4th Place Car
0x1b0 Time Trial 5th Place Name
0x1b4 Time Trial 5th Place Total Time
0x1b8 Time Trial 5th Place Car
0x1bc Time Trial 6th Place Name
0x1c0 Time Trial 6th Place Total Time
0x1c4 Time Trial 6th Place Car

OTHER RECORDS (24-bit/3-bytes values)
0x1c8 Beginner Total Time
0x1cc Mid-Level Total Time
0x1d0 High-Level Total Time
0x1d4 Time Trial Total Time
0x1d8 Beginner Extra Total Time
0x1dc Mid-Level Extra Total Time
0x1e0 High-Level Extra Total Time
0x1e4 Time Trial Extra Total Time
0x1e8 Beginner Mirrored Total Time
0x1ec Mid-Level Mirrored Total Time
0x1f0 High-Level Mirrored Total Time
0x1f4 Time Trial Mirrored Total Time
0x1f8 Beginner Extra Mirrored Total Time
0x1fc Mid-Level Extra Mirrored Total Time
0x200 High-Level Extra Mirrored Total Time
0x204 Time Trial Extra Mirrored Total Time
0x208 Beginner Lap Time
0x20c Mid-Level Lap Time
0x210 High-Level Lap Time
0x214 Time Trial Lap Time
0x218 Beginner Extra Lap Time
0x21c Mid-Level Extra Lap Time
0x220 High-Level Extra Lap Time
0x224 Time Trial Extra Lap Time
0x228 Beginner Mirrored Lap Time
0x22c Mid-Level Mirrored Lap Time
0x230 High-Level Mirrored Lap Time
0x234 Time Trial Mirrored Lap Time
0x238 Beginner Extra Mirrored Lap Time
0x23c Mid-Level Extra Mirrored Lap Time
0x240 High-Level Extra Mirrored Lap Time
0x244 Time Trial Extra Mirrored Lap Time

OPTIONS (8-bit/1-byte values)
0x248 Course Completion Status (many possible values in this offset will affect the "Winner" status on each race and the Extra races being locked or unlocked)
0x24a Key Configuration (Digital Controller)
0x24c Key Configuration (NeGcon)
0x24e NeGcon Center (this is a 16-bit/2-bytes value)
0x256 NeGcon Steer Play
0x258 NeGcon Maximum Rotation
0x25c Selected Course (oddly, count starts from 1, not from 0)
0x25e Selected Transmission (0=automatic, 1=manual)
0x260 Selected Car
0x262 Selected Music
0x264 Unlock Bonus Cars (0=locked, 1=unlocked)
0x266 Unlock 13th Racing (0=locked, 1=unlocked)