The four main parameters that you need to worry about are:
- \$V_{GS\_MAX}\$ or maximum gate voltage. New device should meet or exceed rating of old device
- \$R_{DS\_ON}\$ or Drain-to-source resistance at various \$V_{GS}\$. You will want to compare the old devices' \$R_{DS\_ON}\$ @ \$V_{GS} = \$ some voltage vs the new one at a similar \$V_{GS}\$. You want \$R_{DS\_ON\_NEW} \le R_{DS\_ON\_OLD}\$
- \$V_{DS\_MAX}\$ which is the maximum voltage across drain and source, and effectively how much voltage the transistor can block before breaking down. New device should be \$\ge\$ old device.
- \$I_{D\_MAX}\$ or maximum drain current, which is how much current the transistor can carry when it is fully on without breaking down. New device should be \$\ge\$ old device.
The FETs you are trying to replace are marketed as "High Speed" (whatever that means, its a fairly relative term especially if they are very old), so you may also want to compare switching speeds of the old ones to any new options.
Use a parts distributor's (like digikey, mouser, farnell, newark, etc.) parametric search tool to find a suitable replacement. Further refinements might include package form factor, total power rating (which may not actually be equal to \$V_{DS\_MAX} * I_{D\_MAX}\$ as you might expect), and price.
Most of these parameters are published on the first page of the datahseet, if not, don't use the numbers from the "Absolute Max" section, use the values from the "Recommended Operating Values" or whatever the case may be.
